Shaq is 7 feet 1 inches, 350 lbs, and was an NBA player which means he's in the top echeleon of athletes. Ofcourse Mosley was never going to knock him out. You would only last 8 rounds against Floyd, old or prime, not Pacquiao. Old Pacquiao would knock you out within 8 rounds, and you wouldn't last 4 rounds with prime Pacquiao. Styles make fights and Mayweather's style just doesn't scale.