“Brainy children are not cleverer solely by virtue of having more or less gray matter at any one age,” says Judith Rapoport of the National Institute of Mental Health (NIMH) in Bethesda, Md. “Rather, IQ is related to the dynamics of cortex maturation.”
This is not a recent article as it dates back to 2006. But just give it a read anyway if you want to know how brains of smart kids work.
The full link: Smart Kids Found to Undergo Delayed Brain Development.