Really doesn't matter, depends on the fighter. Forearm muscles, triceps, shoulder muscles and back muscles are what generate punch strength, with forearm muscles playing biggest role in generating leverage.
how does the forearm become the anchor? leverage comes from having a solid base from which to transfer energy from. starts at the well balanced legs with a push off to start the energy transfer, to the hip and shoulder rotation that increases speed and weight behind the energy, to the snapping or punch through of the glove at the transfer point. how are forearms more important in this process?
Ideally, you'd kind of want to connect with as little surface area as possible and have knife hands like the T1000.