Isn’t it volume which incurs greater costs on the organism to regulate itself and surface area which better increases an organism’s capacity to do so? The material which I read in writing this post and a quick conversation with @Bird seemed to confirm this. Unless I’m misunderstanding what you mean.
I thought that was the point of the square cube law and I would anticipate more large cells if it wasn’t so. Cells are so small because of this SA:V ratio rule, where after a certain point, they just don’t have enough surface area to supply their internal structure since costs associated with volume starts to outpace any gain onset by increased surface area. That was where multicellularity stepped in as such a valuable strategy; maintain your surface area and volume, but still get bigger.
I know eukaryotes broke a lot of size constraints prokaryotes face, but that’s because endosymbiosis/the mitochondria provided so much energy for such a small amount of additional costs that the prokaryotic “rules of the game” were broken for a while.
IMPORTANT NOTE: Once again, I’ve realized that I am not the smartest man alive and have been using my terms incorrectly, as I’ve been using “osmoregulation” as a catch-all term for metabolic costs. So Maxonovien is correct, osmoregulation costs do increase with increased surface area. It is the rate of diffusion which increases with surface area, which takes in important resources faster and therefore makes too much volume potentially dangerous - too big of a cell cannot get its resources fast enough without adequate surface area.
In light of this, I guess there are two things that can be a negative to excessive volume:
- We make compound absorption speed be heavily influenced by one’s surface area.
- We add an “inefficiency” debuff of some sorts applied to the organism as a whole, where ATP costs increase to represent the added amount of effort required for resources to flow into the cell. Players can reduce this debuff, or perhaps even mitigate ATP costs, by having a good surface area to volume ratio.
I’m in favor of option two since I feel like it could be a bit confusing for players if they have an organism that should be energetically economic, but still not be producing enough energy because you aren’t absorbing stuff fast enough. It could be hard for the player to determine if the rate of absorption is adequate considering it can already be hard enough to see exactly how much of a compound is at a spot. So I think a more general “inefficiency” debuff would be easier for the player to address.