So although the extreme lower range of modern human brain sizes does overlap that of Homo erectus, their skulls are very different: in H. erectus, the brain case really is smaller in relation to the rest of the skull. (talkorigins.org)
Metabolic acceleration and the evolution of human brain size and life history. (nature.com)
Heat helps free up energy by softening foods, denaturing their proteins and breaking down toxins, Wrangham proposed, which is why cooking may explain human brain size as well as small canine teeth and small guts in comparison to other primates. (scientificamerican.com)