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. Michael, being short, may have caused the others (including Moe Green) to be biased and underestimate him.
I do not agree with that assessment at all. Michael's physical appearance had absolutely nothing to do with the other families not feeling intimidated by him.
It had to do with the Corleones making them believe that they were very weak. They probably also underestimated Michael because in their minds he was not, like Sonny, groomed for the position. He was really looked upon as a civilian and therefore was perceived by the other families as being inexperienced for the position of Don and what he was about to deal with. They took Vito's appointing Michael as head of the family as a sign of weakness. That turned out to be a fatal mistake on their part.
His physical size had nothing to do with how they perceived him and the Corleones.
