So the question becomes, why does this size decrease not decrease general intelligence, or if it left general intelligence alone, what capabilities did it affect? (Maybe visual working memory, given chimpanzees’ superior capabilities in that regard.)
specific brain features don’t necessarily map to well defined categories of abilities.
No, but size correlates fairly strongly with intelligence, in both primates and humans: http://en.wikipedia.org/wiki/Neuroscience_and_intelligence#Brain_size
So the question becomes, why does this size decrease not decrease general intelligence, or if it left general intelligence alone, what capabilities did it affect? (Maybe visual working memory, given chimpanzees’ superior capabilities in that regard.)
Related reading: http://www.gwern.net/Drug%20heuristics#modafinil