Well, Salvo, we will have population system in 1.2 release, which changes a lot when it comes to battles. It makes even diplomacy actually matter as you do not want to get into to many alliances, since that might end with too many wars for your population to support. Plus supply system and some perks also increase unit upkeep so for example if you decide to give army perk to armour, it will increase the upkeep too.