Let's talk about bio load.

[reefwars]

While they use nutrients they do not really remove them from a system just process them and get rid of what's not used, this may be food for other life including other coral . Certain things can not get processed out and eventually need some sort of export out of the system.

As for the size thing , its not always the biggest animals with largest bioload , snails , urchins and sea hares tend to do a lot of pooping lol

I would bet a sea hares waste is ten times as much as fish three times it's size.