Yiddish: If he were twice as smart, he'd be an idiot.

SOURCE: Kumove, More Words, More Arrows.

װען ער איז צװײ מאָל אַזױ קלוג, װאָלט ער געװען אַ גולם!

Ven er iz tsvey mol azoy klug, volt er geven a goylem!

If he were twice as smart, he'd be an idiot!