Man found missing 90% of his brain defies scientific understanding - most of his skull is filled with water:
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(07)61127-1/fulltext
Measuring the volume of a skull was once a valid way to determine intellect. We make fun of people who believed that, but IQ is arguably a worse instrument, and certainly no better.
The problem is you can't measure intelligence any more than you can measure charm or beauty. All instruments who claim to do so are doomed to embarrass the people who use them.