I think that all species have a certain optimal size, which is continually being modified by selective pressure--either bigger or smaller, over time. Presumably a fish's adherence to principles of bioenergents helps it find this size in the best amount of time, but as Dave implied, fish don't stop growing, so the need to "bank-account" resources may be the most influential factor.
JHAP's "return on investment" analogy may be counterbalanced for a need to "put resources into account".
Don't we all face these kinds of questions on a day to day basis?
