Each species will balanced the use of resources between activities
such as feeding, breeding, avoiding predators, and learning how to optimise these resources by using the brain. There are many different evolutionary strategies.
For instance some fish lay millions of eggs while humans have small numbers of
young and use their brain to maximise the survival of each youngster. However
we can be certain that no animal evolves an organ bigger than it needs, and if
conditions change an organ will shrink if it is bigger than necessary. This will
apply to every organ and function and no species will not evolve a brain bigger
than it needs.