I think torben is arguing is that there is no random chance with horses, its just with the heavier you need to just make a hit that shouldnt normally glance to bring it down. There is no 'random' chance, its more of a case of whether you can have the skill to hit the horse dead centre and killing it rather than getting a vague, weak hit that rears the horse.
The advantage of using a heavy horse would be its increased hp to arrows etc. You'd have to find a way to make polearms better vs horses than any other weapon, my suggestion is the ability to couch a polearm on the ground. This would be quite good for spear walls, could mean just a few lined pikes could down a large amount of horses. I dont find heavy horses particularly op atm anyway, I fear an intelligent cav player on a courser or arabian far more than on a cataphract. Heavy horses are just not cost effective if you can play a strong light horseman game...