It is a commonly held belief that brains produce thinking (brains excrete thoughts as the liver excretes bile), and that thinking produces intelligence, which leads to the supposition that bigger brains would produce more intelligence.
Bigger brains would require bigger heads, so people with bigger brains should have bigger heads.
And then you have these guys. Somalians are the 2nd stupidest people on earth and they have these huge lightbulb shaped heads.