80-90% of brain mass are not neurons, but the "supporting" tissue, taking care of things like storing nutrients supply. It is entirely possible to loose that extra mass and still be fully functioning.
If most of the "power supply circuitry" can be stripped without much obvious harm, why isn't it more often? More "thinking" tissue with less support tissue could give clear advantages.
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?
Maybe this person is just one bad head injury from death where otherwise you and I with a similar injury would heal with hospitalization, or maybe just nurse a concussion for a day or two and be fine.
Presumably the giant ventricles are not functioning as proper supporting tissue though. So there's still a great deal of stress the brain has adapted to.
<former brain scientist>
There is another famous case about a person who had a (physically) big head, to the point that people commented on it. Person ends up getting a Ph.D. in math, because hey, you know, big head, right?
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.
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.