Hi @susejdog and @puddleduck
Some animals have bigger brains, and some have smaller brains. However, I think I am right in saying that ours are the most complicated and complex.
As for why, it’s probably to do with the situations animals’ ancestors found themselves in, way back in history. Something caused us to evolve big complex brains, but for other creatures it did them no advantage to have bigger brains, so they didn’t evolve.
It’s also worth mentioning that there isn’t an exact relationship between brain size and intelligence, or even between brain size as proportion of body size and intelligence. That means that, just because an animal has a bigger brain than a human, it doesn’t mean they are more intelligent than a human; and just because an animal’s brain takes up more space in its body than a human’s brain takes up in the human body, it doesn’t mean that the animal is more intelligent than a human. A bottlenose dolphin’s brain is bigger than a human’s, and yet the bottlenoses haven’t evolved to take over; and a shrew’s brain takes up more space relative to the size of its body than a human’s does, but the shrews haven’t taken over either.
Also, we don’t understand everything that is done in every part of the human brain anyway, so it’s not certain that bigger is the same as better!
I don’t think that answers the question any better than Suzi did, but it tells you about some more of the factors, maybe …