Good points from Jim.
All things being otherwise equal, a light gun will kick more than a heavy gun with the same load fired at the same velocity.
A short gun will bruise you because it is more likely not to be in snug contact with your shoulder when you fire - giving a hammer blow rather than a push.
I never use recoil pads unless they happen already to be on a gun. I suffer no bruising with guns that fit me and are correctly mounted - even after a long day in the pigeon hide in shirt-sleeves.
I notice with clients that when testing guns, poor fit leads to bruising - it is most common with short stocks but a 'too long' stock tends to make people mount on the upper arm and get bruised there.
Poor comb fit can lead to bruising of the cheek-bone and a too fat grip and too much reach to the front trigger leads to bruised fingers from recoil.
All these issues are better fixed by a properly fitted gun than by recil pads alone.
All that said, a cartridge loaded to a degree that exceeds the weight of your gun will make life uncomfortable.
