IIRC, the way it works is that they have X people near you, and Y people in reserve. Then each round, more of them move from back up to active (they get in range/active combat).
So, for example, there are 100 guys total, but 20 are in range in the first turn with 80 still in the back, you kill 10, but 20 move over, so now there are 60 guys in the back, 10 dead, and 30 near you.
Basically, the idea is that they send waves at you, so you don't have to fight them all at once, in case you don't have the numbers for that. If you have a huge army, it makes it really easy, but when you have a smaller one, it gives you more of a chance.