Ever Since Darwin, 6: Size and Shape, from churches to brains to planets, 22: Sizing up Human Intelligence

More than 99% of animal species are smaller than we are.
Even among the 190 species of primates, only the gorilla regularly exceeds us in size.

Our size has a lot to do with the development of our brain.
Nature doesn't miniaturize cells. Small animals have fewer neurons in their brains, not smaller ones. Our large size served as a prerequisite for our self-conscious intelligence.

Then, how come elephants and whales have still larger brains and not have superior mental ability? Larger bodies just need larger brains. In fact, smaller animals have more brain per unit body weight than larger animals. The right criterion isn't the ratio between brain-body weights (brain weight increases at about two-thirds the rate of the body if we plot these weights for several animals). It's the ratio between the actual brain weight and the expected brain weight at that body weight. By this measure, we are the brainiest animal by far.

Is this just due to an increase in the body size for our species? No, our brain weight has increased much more rapidly than any prediction based on body weight increases would allow. We are indeed smarter than we were!

Ever Since Darwin: Reflections in Natural History by Stephen Jay Gould