Or from 300 to 1200. Though they didn't pose too much of a problem. As far as I've seen, they only attack if they have a bigger "power" than your army, I have yet to see anything that goes against this (but I could of course be mistaken). It's rather annoying that they make this jump though, I'd rather wish they'd make the opposing armies grow faster or something like that, instead of doubling or tripling in power.
On an unrelated note, I'm loving some of the battle music in the game. Pugilis, Aurora, Ignis and Nixtorm to name some are pretty awesome.