Larger organism sizes are neither inevitably beneficial nor cost free. Costs include the need to move materials further around either the interior of cells or instead around multicellular organisms. In addition is the requirement, with increasing organism size, that bodies have an ability to resist shear forces. First, though, I address the question of just what selects for larger organism size. That is, addressing the question of what are the benefits of being larger rather than what are costs.
Why be larger? Part of the answer stems from complexity. That is, just as greater complexity may be necessary to achieve larger sizes, so too does larger size allow for the achievement of greater complexity. Larger organisms, in other words, are able to do things, such as phagocytosis, ingestion, and physical separation of resource access, with the latter, e.g., such that both body anchoring and shading of competitors may be achieved. These abilities are unavailable to smaller organisms, in part because it is possible to fit only so much complexity in the latter. In all of these examples, however, simply being larger is important, such as in terms of the size of the particles that may be consumed, or the degree of separation between resources that may be achieved. Multicellularity also allows cellular differentiation, which for obvious reasons cannot be achieved simultaneously among the "cells" making up unicellular organisms.
Greater size can also be a reasonable evolutionary response to the achievement of greater size among other species. Thus, prey species may achieve an at least temporary respite from predators, especially ones limited to engulfing, by growing larger while one response to shading by another species is to shade in return, which is achievable, for example, through growing larger (i.e., taller). Tearing up prey rather than swallowing them whole also may require greater size, and certainly greater complexity. Even growing over an otherwise shading individual, such as by vines growing on trees, can require growing larger if there are issues of separation of resources (such as soil and light). A number of organisms also are able to physically separate their resource-gathering functions from their dissemination functions – such as may occur in soil versus overlying air – by building relatively large structures (stalks) that serve to elevate the latter above the former.
There also exist sulfur-oxidizing, nitrate-reducing bacteria (energy source and electron sink, respectively), that are found in marine environments (Thioploca). These bacteria employ sheaths to allow guided cell movement between sulfur rich but nitrate poor sediments and nitrate (and oxygen) rich but sulfur poor overlying water! They form huge microbial mats in sea water, with one such mat reportedly is the size of Alabama (> 100,000 km2) (Schaechter, 2010) :
Sulfides are abundant in the sediment, nitrates in the water column. How to bring the two together? Thioplocas solve the problem by making tubular sheaths that stick out from the ocean's sediment. These sheaths can be as long as 15 cm. Inside them, filaments of the organisms glide up and down, gathering sulfides below and nitrate above. Thioplocas chemotax towards nitrate. They absorb it and transport it downwards, to where the sulfide is abundant, at speeds of about one centimeter per hour. In other words, thioplocas take an elevator to work… The thioploca's gelatinous sheaths get to be huge, walls as thick as 1.5 mm and as long as 10-15 cm, thus readily seen with the naked eye. The bottom 5-10 cm is buried in the sediment, thereby firmly anchoring the sheaths and making the mats very stable. The sheaths lie flat along the surface and look yellowish, according to those who have seen them. Each sheath may be shared by as many as 100 individual filaments, each of which can glide up and down independently.
Avoiding being shaded by other individuals or, individually, gaining access to more nutrient rich streams also can be viewed as benefits of simply being larger. This is an aspect that we have already considered in terms of microcolonies in biofilms, where greater size in terms of distance from the anchor point to a solid surface can result not only in avoiding being shaded by other individuals (in this case microcolonies) and indeed may allow for access to nutrient streams that otherwise would be beyond reach. Marine sponges accomplish a similar endpoint, perhaps especially in comparison with colonial choanoflagellates where greater size means a thrusting of the sponge body further from their benthic anchor points, thereby quantitatively or even qualitatively gaining greater access to nutrient-carrying streams of water.
An interesting way of viewing advantages versus disadvantages of displaying greater size comes courtesy of Dawkins (1999) . Here the basic premise is that cell division can be considered to contribute to organism growth versus organism reproduction. At an extreme, all single-celled organisms display no "growth" as a consequence of cell division but instead display all reproduction. At another extreme are very large multicellular organisms which contribute a great deal of their cell division to growth, but a relatively small amount to reproduction. These differences, from an evolutionary ecological perspective, can also be seen in terms of the principle of allocation, where the larger organisms are channeling energy into something other than reproduction, and therefore, for a given amount of energy obtained, are producing fewer offspring. In absolute terms, however, the larger organisms may in fact be individually producing far more offspring, except that they are employing far more energy as well as far more time to produce those offspring (re: conflict between rates of replication and replication yield). Clearly this extra effort would not be worth expending except that what the larger organisms are accomplishing is an ability to gather resources that are unavailable to more reproductively oriented organisms. That is, some resources require greater prior investment in order to exploit, and this greater investment may be viewed in terms of requiring a greater allocation of cell division to growth rather than directly to reproduction ("more capital investment is required to be laid down before returns on investment are sought" as Dawkins puts it). The result is selection for an evolution of greater organism size. Quoting more fully (p. 254):
The many-celled body is a machine for the production of single-celled propagules. Large bodies, like elephants, are best seen as heavy plant and machinery, a temporary resource drain, invested in so as to improve later propagule production… In a sense the germ-line would 'like' to reduce capital investment in heavy machinery, reduce the number of cell divisions in the growth part of the cycle, so as to reduce the interval between recurrence of the reproduction part of the cycle. But this recurrence interval has an optimal length which is different for different ways of life. Genes that caused elephants to reproduce when too young and small would propagate themselves less efficiently than alleles tending to produce an optimal recurrence interval. The optimal recurrence interval for genes that happen to find themselves in elephant gene-pools is much longer than the optimal recurrence interval for genes in mouse gene-pools. In the elephant case, more capital investment is required to be laid down before returns on investment are sought. A protozoan largely dispenses with the growth phase of the cycle altogether; and its cell divisions are all 'reproductive' cell divisions.
Another way in which growth may be distinguished from reproduction is in terms of "growth" rates versus yields as well as expediency versus economy. Here body growth is the less expedient strategy, involving a delay in reproduction during which processes other than reproduction are emphasized. The result, ideally for the economical organism, is a greater yield of progeny surviving to the next generation. Similarly, in emphasizing reproduction over growth, a lineage is potentially emphasizing short-term gains over long-term success. The idea that bodies, in the course of growth, must mature in order to reap the utility of emphasizing growth (yield) over reproduction (rate) is equivalent to considerations that phages (see Virulence: Examples of Virulence Evolution: Phage Virulence) or microcolonies (see "Cooperation: Specific Circumstances: Biofilms and Cooperation") too may benefit from emphasizing growth yield over growth rates, economy over expediency, particularly when dissemination either occurs later or is more profitable following extended incubation.
A cell's minimum size is defined by its need to carry ribosomes and other gene-expression machinery as well as its need to collect resources from the extracellular environment and then transduce those resources into the basic building blocks of anabolism. Viruses can achieve smaller sizes than cells because they steal these functions from their cellular hosts rather than carrying and encoding basic cellular functions themselves (though note that not coding basic cellular functions is less true for very large viruses than for smaller ones, an exception of sorts that "proves the rule", that is, that encoding basic cellular functions can serve to limit how small an organism can be). A cell's maximum size, by contrast, is controlled by its ability to move materials about intracellularly. Size limitations can occur particularly because the default means of such movement, that is, diffusion, is too inefficient to service too-large volumes, either in terms of movement across those volumes or in terms of movement into those volumes (i.e., across the plasma membrane).
To some greater degree, greater size can be achieved without a requirement for extensive, ongoing movement throughout the cell. One strategy towards these ends is by making some cell parts relatively inert, such as one sees with various cytoplasmic storage devices (e.g., inclusion bodies) and as perhaps best exemplified by the central vacuole seen in plant cells (though the plant cell still needs to move RNA transcripts, proteins, and various building such as of membranes about the cell); Thioploca, the sheathed bacteria that move between sulfur and nitrate sources, too display "huge" vacuoles. Even with such mechanisms, however, there exists a tradeoff between achieving greater size and remaining both structurally simple and metabolically dynamic. See the section titled "Endomembranes" for continued discussion of how larger cells can sustain intracellular movement.
This problem of substance movement about bodies increases as organisms grow to yet larger sizes. One solution is to avoid separating cells such that "multinuclearity" is achieved without cytoplasmic division. This solution actually is achieved by many multicellular organisms that retain cytoplasmic bridges between cells, though is most fully developed in certain coenocytic (or equivalent) algae (i.e., Vaucheria), fungi, and slime molds, all of which are organisms that exist as truly undivided but nonetheless multinucleated cytoplasms (termed plasmodia in slime molds; indeed the malarial parasite, Plasmodium, displays a similar morphology in the course of mitotic division). The advantage of this solution is that within-organism movement consists of an extension of within-cell mechanisms of movement, e.g., cytoplasmic streaming. The disadvantage is a combination of limitations on organismal differentiation (single cells cannot differentiate to the same extent that two or more cells can), organismal size (since mechanisms of intracellular movement are only so efficient), and that any breach in the organism is a breach in its collective cytoplasm.
Coenocytic organisms possess multiple nuclei within a single cytoplasm that otherwise makes most or all of the organism's body. This is in contrast with multicellularity or instead with microscopic unicellularity. The above video provides a brief discussion of this strategy.
Multicellularity is the solution that contrasts with producing larger organisms with ever larger cytoplasms. With multicellularity, however, an organism is placed right back to the original concern that movement of materials around the organism is inefficient through diffusion alone, but now without the potential to employ intracellular mechanisms to enhance that movement. Thus, the evolution of greater organism size may be most easily achieved without multicellularity since the material-movement problem continues to have approximately the same solution as the organism grows larger, whereas with multicellularity there exists a limitation on total organism size (i.e., small), or shape (i.e., thin), until more efficient mechanisms of extracellular movement of materials evolve, e.g., such as the development of tubes, various kinds of pumping devices, or motile cells capable of carrying materials between more stationary cells.
All three of the above mechanisms can be seen, for example, in sponges, a simple form of multicellularity which nonetheless is able to attain large sizes. Sponges can do this because they employ flagella to pump water through passages along which resource-collecting choanocyte cells reside, and sponges also possess amoeboid cells that function to transfer resources collected by choanocytes to other cell types found within the sponge. Note, though, the requirement for cellular differentiation, such as choanocyte versus amoeboid cell types, in order to surmount limitations on the size of multicellular organisms that otherwise stem from inefficiencies in the movement of materials.
With larger size also comes a problem of exposure to greater shear forces, thus necessitating not just greater complexity but also greater resistance to breakage. This resistance may be accomplished through either intracellular or extracellular support systems, or combinations of both. Intracellularly one finds cytoskeletons. External to the plasma membrane, cell walls in various forms abound. They are so common, however, that clearly their existence predates evolution of larger size. Thus, cells walls, such as the cellulose cell walls possessed by plants, can be viewed as something of a preadaptation, a structure that provided support and perhaps some degree of protection in unicellular and small multicellular organisms but which only later was co-opted to provide support for larger multicellular structures.
Since the most common mechanism whereby even greater organism size has been achieved is through the transition from unicellularity to multicellularity, so too the cytoskeleton can be viewed as a pre-adaptation that supplies shear resistance with greater size, e.g., such as in animals. Animals, however, have also evolved extracellular, collagen-based connective tissue that augments the intracellular cytoskeleton. Thus, one can view animals as a series of alternating extracellular and intracellular solutions to achieving shear resistance. Resisting being torn apart by water currents, other organisms, or wind thus are inevitable problems of gaining in size, but for which there exist numerous solutions, many of which are built upon mechanisms that contributed as well to the structural soundness of ancestral unicellular organisms.