system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are humans vastly more intelligent than even animals with vastly larger brains like elephants or whales?
gpt
Neuroscientist here, also with some experience in intelligence research.

Generally, is believed that brain mass to body mass ratio is a loose indicator of cognitive ability. 

So, what explains deviations from that tend?

One explanation is highly developed but very specific abilities. Humans are particularly specialized for general intelligence, but other species may rely on highly specialized abilities like extremely high visual acuity, extremely detailed spatial memory, or more complex auditory systems that can be used for echolocation. These might drive up brain mass without increasing overall cognitive ability very much.

Another may be metabolic. Factors like the need for insulation or caloric efficiency may drive up brain mass as well. 

And in at least some species, degree of brain folding (versus smoothness) correlate with cognitive abilities.

So, long story short, cramming in more neurons makes brains bigger, but a lot of other stuff CAN also make brains bigger, which is why brain/body mass ratio is not a perfect indicator.