Currently the game developers have set a medal cap calculated off of your base score:
Medal Cap = Base Score x 10 + 100
In the example below, the calculation would be: 699 * 10 + 100 = 7090. This player is currently medal capped. Even if the player wins PVP matches or successfully defends his base, no medals will be awarded.
Note: There are plans in the works to alter this equation and/or factor the Medal Counts into PVP match-up selections. Stay tuned!
UPDATE: The medal cap equation is now:
Medal Cap = Base Score x 10 + 300
Apparently this happened by accident, but it didn’t break anything with matchmaking, so the developers didn’t change it back.