Why does the intelligence of an animal correlate with the brain to body mass ratio more closely than to the mass of the brain itself?

It seems counterintuitive that a large animal with a large brain is likely to be less intelligent than a small animal with a medium size brain. It would make more sense if the intelligence of the animal correlated simply with the mass of the brain, since there is more tissue to use for cognition – yet it is the ratio of brain mass to body mass which seems to matter more. Why is this?