This reminds me of the massive "myth of the cavalry charge" thread we had around when MTW was released. There are some differences between knights and heavy cavalry in ancient times, but I would see legions vs heavy cav as rather like Saxons vs Normans at the battle of Hastings. With a solid mass of disciplined heavy infantry (a shield wall or whatever), I can't see cavalry simply running them down. This is just armchair conjecture - some history buff may tell me (as they did about knights) that horses were trained to charge into spears, but I just don't find that a particularly plausible model of how cavalry fought good heavy infantry.
Actually the battle of Hastings is a very good example of what happens when heavy infantry face cavalry. The Saxons held firm for hours, the Normons did charge but their horses refused when it came to the shield wall.

IIRC the way the Normons won was by hit and run attacks and feigned retreats. Eventually a good part of the Saxon infantry chased them down the hill and broke away from the main army, without their formation they were ridden down and slaughtered.