They say that size doesn't matter, but try telling that to bacteria. Most are very small, for they rely heavily on passive diffusion to ferry important nutrients and molecules across their membranes. To ensure that this happens quickly enough, bacteria need to ensure that their surface area is large enough relative to their volume - become too big and they won't be able to import enough nutrients to support their extra size.
These constraints greatly limit the size of bacteria. The larger ones solve the problem by being extremely long and slender, or by using an internal compartment called a vacuole to push their insides to their outer edges. But even these solutions have their limits, and the simple fact is that most bacterial cells are far smaller than those of the more complex eukaryotes - a group that includes all known animals, plants and fungi.
But one group of ...
