Brain size relative to body mass probably has something to do with intelligence.
A blue whale has a massive brain but lots of that is just used up to recieve and send signals to the vast amount of tissues it has. Likewise a bigger man has a larger brain than a smaller one but he is not necessarily smarter.
If you control for body mass you control for the non-intelligent parts of the brain that recieves, for example, sensory signals or sends out various signaling substances into the blood.
