Recently I have been trying to increase my team's power and noticed that I level up much faster when playing the later levels than when I play the early ones. I know that in pokemon it gets harder to level up because of the exp increase from level to level, but in Fire Emblem: Awakening each level only requires 100 exp. So what determines how much exp you get from an enemy?
The experience gain is based on several factors. If you hit an enemy, you obviously gain some experience. If you defeat them, you gain significantly more. The amount of experience these actions give you are based on your character's level in relation to the enemy unit's level.
A level 1 promoted unit is essentially a level 21, if that helps these make sense.
This question has been asked many times on GameFAQs, and I find this thread to be particularly helpful and informative.