If an army has larger numbers they should win. That being said, there is a cost to holding too much land or moving in larger groups that may make it so smaller forces have something to do. In the current campaign, Order is a ghost town (NA). We are consistently losing, and without some serious infusion of players, it seems that this may become ad hoc 2 realm game.
1. Over Extension is expensive and difficult; Taking a fort, or any other asset should require some gold to maintain. The further from one's home base the more it should cost. The cost could be reduced by maintaining supply lines. Those lines should be visible (i.e. caravans) and subject to attack. If the cost cannot be paid, the keep/fort will begin to deteriorate, lose its guards, maybe even have an occasional uprising of NPC's;
2. Map indication: there should be something on the campaign map that shows larger forces. When ten or more players of the same faction are moving within the same parcel, put a flag there or something. Make the marker larger based on the actual number in the parcel.
3. Underdog Rewards: Prop up the failing faction in campaigns with rewards based on the population disparity; and
4. Mercenaries? and other NPC threats: seems someone may have mentioned this already. Lower pop faction should have access to mercenaries that can be purchased to attack targets. In addition, being the wealthiest "nation" should also result in Bandits, monster invasions or whatever, putting additional pressure on factions holding substantial lands.