The UESP article on leveling says that the rate at which combat skills level are based on "raw" damage either dealt or received.

But what is "raw" damage, anyway?

raw damage is your overall damage output. Like if your sword does 10 damage, and strength boosts your damage by 5, your "raw" damage is 15. This is the value BEFORE things like armor and other effects are calculated to reduce or eliminate the damage.

Note: I was pulling numbers out of my... head. So the numbers are all fictional and not related to any actual gameplay.