No, an elephant needs a larger brain to run its larger body. Humans have enough brains to run our bodies, plus "extra" with which to think, in a manner of speaking.
I've always thought the metabolic needs must have been just as driving (specifically, being carnivores). Note: not a dig at vegetarians, we're talking about long before the proliferation of tofu and quinoa.
"A gram of brain tissue takes 20 times more energy to grow and maintain than a gram of tissue from the kidney, heart, or liver"
"A gram of brain tissue takes 20 times more energy to grow and maintain than a gram of tissue from the kidney, heart, or liver"
http://news.harvard.edu/gazette/story/2008/04/eating-meat-le...