A perennial question is why prokaryotic cells are so small,
and the typical answer is that they require a large surfaceto-volume
ratio to support their internal biochemistry.
However, Koch estimated how large a cell could be if, like
the enormous symbiotic bacterium Epulopiscium fishelsoni
[13], it only divided once per day and depended solely on
diffusion in a nutrient-rich environment [4]. His answer
was that a bacterium could be over 800 mm in diameter!
This implies that limitations on the sizes of more typical
prokaryotes are not due to the ability to take up nutrients
per se but arise from the competition for nutrients, a
competition won chiefly by smaller, faster growing cells.
The lesson is that although diffusion-limited nutrient
access might affect cell size, it does not by itself explain
why bacteria are mostly small.