This goes to show the insane levels of plasticity that our brain has, and the great importance of the cortex.
I wonder at what age he developed this condition and if he felt any symptoms/signs while it was developing.
OTOH if the supporting tissue id not excessive, what are the circumstances when it's important? Will this person have hard time holding breath without fainting? Fasting without fainting? Act well under a high prolonged intellectual load?
You'll reach thermal and energy constraints.
Or, only half-joking here, you'll fall prey of a terrible depression and withdraw from all human company.
Turned out that the head size was due to a hydrocephalic condition-- the person barely had any brain tissue, most of the contents of their skull was fluid.
Yes, the brain is pretty amazing.