Kalt:

As soon as trade starts up, AND (as DrNo suggested) the AI has a part in this, conquest is seriously constrained by any war economically. If you have to hunt down his trade links, because yours and his mutually block each other, you get a big hit on your trade.

Imbalances only come from the player as the sole trade imperium on the map, thus still able to get tons of cash even in times of war.