Elephants have much bigger brains than humans. That doesn't necessarily mean they are more complicated. Maybe they are, but elephants are obviously not able to do, intellectually, nearly as much as humans. Generally, brain size scales with body size. Humans are unusual in that we have brains that are much too big for our bodies.

Structurally, our brains are like those of other mammals. What makes them too big for our bodies is the cerebral cortex. Now, I don't want to mislead anyone by appearing too confident. So let me say that this was pretty much the extent of my knowledge on comparative brain anatomy. I believe that the structures one is aiming for here, are simply larger in larger animals. Besides, when operating on a human, you can always ask them how they feel when you apply a current to a part of their brain to make sure you got the right bit. Standard practice, actually.