There are huge bonuses to damage and armor from leveling up, hence why getting one shot while only scratching the enemy if he is max level while your level is low. In some matches while I am queued with similar levels players, grunts can be killed in 3-4 shots, on the other hand, if queued with high level players, I lose 25% of my ammo only to kill 2 grunts ( if I survived that is ).