Har för mig att jag har hört att för människor/apor gäller generellt större hjärna=smartare.
edit: wiki: When comparing different species the ratio of brain weight to body weight does present a correlation with intelligence, though the actual brain weight has little or no effect. For example, the ratio of brain weight to body weight for fish is 1:5000; for reptiles it is about 1:1500; for birds, 1:220; for most mammals, 1:180, and for humans, 1:50.
http://en.wikipedia.org/wiki/Neuroscience_and_intelligence#Brain_size