True, but it bears repeating that the surface of the brain is where the magic happens, and that's only casually related to mass. Most of the mass in the center

True, but it bears repeating that the surface of the brain is where the magic happens, and that's only casually related to mass. Most of the mass in the center is for support structures and a place to thread interconnect fibres through. All that stuff still has to happen for a hydrocephalus patient to function, but there's a lot more mass than technically needed. Intelligence decrease in hydrocephalus comes from the tissue trauma associated with it, not necessarily the decrease in overall mass itself. Unexplained might be a bit of an overstatement, but there is certainly a lot to be learned from examining a little closer how the brain deals with damage.