Re-organization more important than changes in brain size
A new finding points to brain reorganization, rather than brain size, as the driver in primate brain evolution. Data from 17 anthropoid primate species (including humans) across 40 million years has found that around three quarters of differences between the brains of species of monkeys and apes are due to internal reorganization that is independent of size. The prefrontal cortex in particular appears to have played the biggest role in explaining the evolutionary changes in primate brains.
http://phys.org/news/2013-03-organisation-trumps-size-primate-brain.html