Just some thoughts …..
[list=1](1) F = m x a
(Force = mass x acceleration)[/list=1]
Force required to move mass m (your fist) with acceleration a. This is just a really simplified model.
Since a = rate of change of velocity with respect to time, we can also write:
[list=1](2) F = m x dv/dt
Where dv/dt is the rate of change in velocity.[/list=1]
So if you imagine accelerating your fist, from rest, then to achieve the greatest force you want to either get to the highest speed possible or do it in the smallest time interval; preferably both.
But what happens when you make contact? Well the idea is to try and transfer all the momentum (or energy, used interchangeably although they are different things) of your punch into your opponent.
Ideally you'd transfer 100%, which brings your fist to a rest and all the momentum/energy is transferred into your opponent. Imagine the executive "cat's cradle" toy, where you've got a row of steel balls suspended by string from a frame. You draw one ball back, it hits the other balls and the ball on the other end is flicked back. Well that what you want your punch to do.
We can re-formulate equation (2), because momentum p = m x v (mass x velocity). So:
[list=1](3) F = dp/dt[/list=1]
The greater the change in momentum the greater the force imparted to the opponent. So if the opponent is at rest, or coming towards you, then the momentum of your punch will have a greater effect than if he was moving away.
If we look at things from an energy viewpoint. The kinetic energy of your punch, converted from your muscular potential energy, is given by:
[list=1](4) 1/2 x m x v2
(half x mass x velocity squared).[/list=1]
The faster your fist is moving, the more kinetic energy you will impart to your opponent at contact.
"But what about heavy weight boxers?", I hear you ask; "they can punch harder than light weights". Probably many reasons for this, but I can think of 2 off the top of my head:
[list=1]1) I'm only describing the fist, propelled by the bicep/tricep muscle pair. In reality you're punching with your whole body and therefore the larger the body mass, the more momentum you can deliver.
2) There may be a limit to how quickly one can move a fist, and it may have nothing to do with how big you or your muscles are.[/list=1]
So for a given mass, I think it may be better for someone to work on their punching speed rather than just get bigger. As has been said before on this forum, there are many kinds of speed in WC (or any martial arts for that matter).