The suggestion is that the female brain is able to develop more conncetions than the male brain or that the connections are more efficient.
By that I mean using less tissue to get the job done: a bit like a road network that has the most efficient use of building materials to get you where you need to go.
I've always suspected that men think in straight lines where as women can think around corners (maybe one of my less scientific ideas).
total size is actually less important than surface area.
If the brain were like a grape, size would be the only factor. Instead, though, the brain is like a raisin with ridges and valleys. The more wrinkles, the more surface area. A small but very wrinkly brain could easily have more surface area than a brain many times it's size.