Why does one punch from McClusky break Michael Corleone's jaw and do so much damage

They probably called him "One Punch" Mclusky back in his boxing days.

Depends on the hit. In any case, breaking a jaw isn't a cumulative action: more punches don't wear down the jaw until it breaks. A jawbone will break depending on the force and angle of the hit, not how many punches may have landed before that.

McClusky was a big guy and a seasoned cop. He probably spent a lot of time "discussing" things with those who "resisted" arrest.

Mofo had a big ass fist and could swing it really hard.

i don't think it is all that uncommon at all.

He had such a strong punch because of a lifetime spent preserving his bodliy fluids.

You ever been punched in the face by Sterling Hayden? Six foor four-and-a-half inches, and he served in both the OSS and the Marines. He even parachuted into Croatia and spent time as a guerilla there during World War II.

I'm not surprised Michael's jaw was broken. What surprises me is that he had any face left at all.

Boxers normally retired with several health problems because of the punches, Mickey Rourke aparently got desfigured after sparring with a profesional boxer

One thing is a punch from an average Joe and something diferent is a punch from a profesional boxer who hit you, been angry and with all intention to hurt you, while you are restraint

Just watch regular people trying to fight with profesional boxers they get F**ed up pretty quickly