In a battle that looks something like this:

Attackers:
2k Archers

VS

Defenders:
2k Workers
250 Ballista
1 Battering Ram
10 Catapult


The result ends in a Defender victory with ~150 Ballista and 10 Catapults left.

Though, if you add 1 Cataphract or 1 Cavalry to the mix, Attackers will kill everything with ~500 Archers left.

Why does an extra horse running around on the battlefield tilt the outcome so harshly?