expression
big heads have little wit
Proverb meaning that large, physically imposing people are often slow-witted or lacking in intelligence. Used humorously or critically.
彼は体は大きいけど、大男総身に知恵が回り兼ねって言うからね。
He's big, but as they say, big heads have little wit.
A Japanese proverb. The exact origin is uncertain, but it reflects a traditional folk belief that large people are slow-witted.