There has been many arguments where people have argued about size. Size does matter in hand to hand combat, and that is the truth. However, between the untrained, size doesn’t matter if you can’t land any blows. Despite this, heavier people tend to punch harder than lighter people.
The power difference is due to higher weight. Mass is part of the equation for force, and bigger people can punch slower while generating the same force as a lighter person. A heavier mass also makes them more balanced when pushing a lighter object. Regardless, shifting bodyweight is a big part of the equation. There will be some extremely pillowfisted heavy individuals while some light individuals hit like a truck. Regardless, if you are heavy, out of shape, and weak, then you should not mess with an athletic lighter person with decent strength. When you are unathletic, being big will not give a better chin either.