If you have too many heroes on the battlefield, your army gets confused on who to take orders from.
- For each hero you field more than the number of your enemy, your army gets an attack speed debuff.
- Attack speed debuff = 0.5 * (n^2) % , where n = the number of heroes you have on the field more than your enemy
So, if you have 1 more hero than your enemy, your army gets 0.5% attack speed debuff. 2 more heroes is 2% debuff. 10 more heroes is 50% debuff.
The idea is to make PVP less about just fielding every single hero you have, and making it more about only choosing the heroes you want for your strategy.
Another side effect of this is it would help the seiging issue we see now with defenses being useless. Since an attacker fields something like 10 heroes and the defending garrison has 0 defenders, the attacker would suffer a 50% attack speed debuff.