I also think that besides the fact that Michael did not grow up being groomed as a Don as Sonny might have, the other Dons did not feel intimidated by the short and skinny Michael. Sonny was tall and muscular, with his temper, he was very intimidating. Vito proved throughout his Donship that he should not be reckoned with. Michael, being short, may have caused the others (including Moe Green) to be biased and underestimate him.

Nothing against short people, just a thought. I'm on the short side myself.