So, I know the rules have a section for if I'm purposely trying to push someone back and not hurt them... but what about knockback as the side effect of a massive attack?
I imagine size would play a role, much in the way the adjusted unarmed attack rules add a size modifier. It is after all more like that a jammed industrial robot knocks someone back than my typical 2-foot tall scurrier. However, SOM should also play a role. At 20 a person would be fairly jacked. At 40 I imagine punching you through a wall is just what he does.
Perhaps there's a rather simple solution to this: club or unarmed attacks that deal damage over a threshhold also inflict knockback by how much they exceed it?