While testing defensive formations, I have noticed that in many cases AI breaks its formation with no apparent reason, sending its units to charge my lines piecemeal. I struggled with this problem, trying to figure a way of forcing AI to keep its form. Then I tried different unit sizes. On small size AI charged. It charged on normal. It charged on large. But it didn't charge on huge; on huge it kept its form.

Now, if AI checks that a unit seems weak enough for a frontal charge that'd break the unit (as it tries to do with a general), why wouldn't this apply also on normal units? And what if the checks that are made check the manpower of the unit to be charged, but not the unit scale the game is being played on?
Wouldn't that equal as:
Small unit size, 20 men unit (although full strenght, in neat order, in the middle of players line) vs a full-strenght AI unit => AI perceives the situation as: "only 20 men, full-strenght gen. bodyguard/full-strenght swordsmen will break it instantly"
Huge unit size: 160 men unit (full strenght, in neat order, in the middle of players line) vs a full strength AI unit=> AI perceives the situation as: "damn, 160 men, even a full-strenght gen. bodyguard/full-strenght swordsmen wouldn't break it"

That is, it could be that AI checks the percentual strenght of its own unit against the raw soldier count of the players unit.

I haven't tested this conclusively, but I am pretty sure that this is the case.