There's a way to fix this. Although I'm not sure how hard it would be to code.
If the spawn rates were limited to the ratio of the actual troop numbers. For example: Wtf_Pwnzors army of 500 vs. OmfgRoflStomp's army of 1000. The spawns would be limited to a 1:2 ratio no matter how many people sign up to fight for either side.
Winning?