This used to really bother me before I increased the battle size. If its an option, its worth sacrificing graphics, grass, shadow effects, etc., for a higher number of soldiers on-screen. The lower your battle size limit is, the more reinforcement waves you're going to get; which absolutely sucks when you've just broken through the enemies' line and ripped them all to shreds, only to have 20 more of them suddenly spawn right on top of you. This hurts a lot more playing as infantry, if you're cav you can move away from the spawn area in time to avoid being surrounded.