it's dependant on your tactics skill and basic odds.
For example, if you have 100 men versus 200 of the same strength, the enemy will have a higher battle advantage and have more men spawn compared to yours, maybe 75 vs 150.
your tactics skill can mitigate this effect. with 4 tactics you will prolly find your self fighting them with 90 troops vs 125, much better.
Battle advantage is what this is. Worse the advantage is (negatives) the more troops the enemy gets to field at a time and the less you get to field at a time.