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.
