well, size isn't the issue for brains, if I'm not mistaken it has to do with a few factors, smaller gaps in the synapses between brain cells for instance, larger number of connections between the brain cells for greater efficiency, brain chemistry even.

There has recently been an understanding that birds and possibly dinosaurs though having smaller brains are or were actually much smarter than anyone ever knew for this very reason.

Not sure why the human brain is so inefficent given it's size to use ratio compared to other animals, but I'll hazard a guess that perhaps linked to Progeria is a normally dormant archaic gene in the human genome for brain growth that somehow got activated causing the brain to form more according to the efficient but older reptilian brain growth pattern to some degree.

To some extent the process of learning and practice in of itself forces the brain to create and grow greater connectivity in areas related to the task. It would appear to me that the most natural way to stimulate brain growth could stem from creating exercizes and cirriculum designed specifically to enhance connectivity. It also points to rote falling out of fashion in education being actually a really poor idea, because rote is how the brain makes more efficient connections. It's probably for instance why a minstrel in the middle ages, sort of the Reuters of their day, could hear and immediately memorize thousands of songs that they could in turn pass along and sing with accuracy having just heard it once from another minstrel. All that practice, focus on memorization, before most people recorded thier memories in written words... if anything our brains have atrophied, the nurturing of connectivity in the brain in education pretty much limited to the students who opt to play sports, music, and rarely visual art (since so many art teachers reject discipline and practice for creativity which is mostly banal)