the punch that broke Michaels jaw??

Al Pacino was about 5'4 and 130 pounds soaking wet in his prime, a real scrawny man and Michael was played by him, McClusky was around 6'4 or 6'3 and probably 200 pounds even in his old age and was portrayed in the movie and book as being very large and imposing, a man with a boyish body type that gets punched by another man in the face is going to be hurt like hell, hence the broken jaw.

This why they have weight classes in Boxing and MMA because big guys fighting guys that are much smaller and would be a disaster, Floyd Mayweather for example wouldnt like a second against Evander Hollyfield because he is 145 and Hollyfield is over 200 pounds easily.

Scene really shows you how much of a runt Michael is, but his intelligence and ruthlessness makes him dangerous, more so than Sonny and McClusky combined.