To make things easier, let's have two clones who are completely identical in every way you can imagine, except for the difference in physical strength. Man #1 can lift 2 tons like Louis Cyr. Man #2 can only lift 200 lbs. If they both punched a punching bag with all their strength (assuming they both punched at the same speed), would there be an apparent difference in force, or the kinetic energy?
Your question is a little rigged: the speed at which someone punches (all the way through the target, mind you) is the primary manifestation of their strength. If they accelerate the bag equivalently (ie, punch with the same speed), there is no difference in the force/momentum transfer imparted to the bag.
The total power delivered by a punch depends on several other things apart from the physical strength. A good martial artists always know this thing and generates his power from the twisting motion of his hips , shoulders and his arm whereas ordinarily the strength of a punch just comes from your arm and is not that effective.The power delivered also depends on your speed. Try searching the "one inch punch","six inch punch"