If it doesn't help, and uses more energy, then it won't get kept unless it's an inevitable side effect of something helpful. That was my only basis for "larger (of same type) => more intelligence".

I don't really know anything about this topic. My claim is essentially a tautology that may not have much practical application.

Some trivia (not directly related to my original claim) I found at http://en.wikipedia.org/wiki/Brain_size:

[brain size vs. body size in mammals] follows a power law, with an exponent of about 0.75

the "average" brain of mammals taken as a whole, but each family (cats, rodents, primates, etc) departs from it to some degree, in a way that generally reflects the overall "sophistication" of behavior.

Primates, for a given body size, have brains 5 to 10 times as large as the formula predicts