I'm afraid this sort of thing is actually already taken in charge.
If your army is superior than 300 men, you'll have to pay them (sort of upkeep, representing food consumption and salary).
Garrisons have it easier, since they're not moving, so they don't need as much pay, since they're not working.
What's wrong with fielding huge armies ?
I'm more astonished by the fact every warring army has full heirloomed plated armor for all melee players. There's just no diversity in strat battle equipments, if you can afford the cost.
Can't wait for a couple more theme strategus battles, thoses were really way more enjoyable.