Naturally limiting army size/movement

I have a hex game of territory domination using armies. Players build villages, which in turn can produce troops, and they move the troops around on the map to invade and claim territory. There are four hex types with associated resources and movement costs. There is a worker placement system which limits the amount of troops a village can produce in a single turn (so 4 workers = 4 troops) and which governs resource production.

The players build some troops, pay an activation cost for movement points, and spend those points moving across the map. If they enter a new hex, there may be a small battle with NPC units which can give rewards needed to upgrade. As more villages are founded, the player collects more and more resources. A single resource governs all player actions, although actions (including building certain units) cost additional resource-types.

My problem is that towards the end of the game when the players are established, they have large resource production which is necessary for the game to proceed normally. HOWEVER. It is currently possible for a player to halt all other actions for a round (we assume his position is strongly defended at this point), and spend each turn building troops in a single location. The next round, he can move this entire army using the next turns resources. Alternatively, a player could build smaller armies (normal play), and have them converge into one mega army, and then move this around.

This breaks the game.

I have a running mechanic in the game which says that if you can pay for more than one of the same type of action in the same tile, you may do so in the same turn. So you can build units up to the worker placement value in the same turn (here I limited it because having a mega army pop into existence is dumb thematically and bad for game-play, but its still tied to an existing mechanic so not totally arbitrary), and you can move multiple units by paying 1 activation resource for each unit at the same time (move 5 guys, pay 5 activations).

Now, I can do a number of things, but I really don't like arbitrary limitations in games and would like to avoid them as much as possible. "You may not have an army larger than X" is not as good as "Having an army larger than X is going to be prohibitively costly because Y." So for example, I could say that if your army is larger than X, then you automatically clear the tile of NPC encounters (losing a number of potential bonus resources in the process). This solves the problem of steam-rolling early encounters, but still gains the player the tile. And it does not solve the problem of PvP, which is the core of the game. I could also put some kind of total-turn limit on the individual town production of units, but I cant see how to do this organically and it requires additional state-tracking which would be very time consuming in such a unit-heavy game. It also puts a hard limit on units, such that a player cannot skip units now in order to build some later, which I want very much to be unrestricted so long as it is not abused. Finally, I would like to avoid phasing of the action rounds (where you separate and restrict the player actions into distinct phases... this should be a sandbox as much as possible).

Any ideas? Much appreciated.

PS: I'm not 100% sold on my movement system, but it does have a lot of advantages that I like and I can mesh it with a number of other mechanics..