I agree about the marketing component. But I wonder whether the brain's executive substrate is really "smaller" than the instinctive or emotional substrates. Isn't the cerebral cortex larger than the older parts of the brain? Or where exactly do you think the executive function is found? In speaking of size differences, do you mean to speak of power ones? So you'd be saying that the instincts and emotions have more power over us than reason. That would make more sense, I think. The size doesn't matter much since there are animals with much larger brains than us but that aren't as intelligent.