Well, I am lowering mass but increasing charge strength now, as per ASM's suggestion. Perhaps that will work better. We shall see.

You are correct, properly prepared Roman armies were highly successful against the Parthians; for example, against Pacorus, the numerous Roman slingers inflicted heavy damage on the Parthian heavy cavalry, letting the Romans kill Pacorus and rout his army.

I am seeing a potentially similar outcome here in MP battles. Prepared armies of large kingdoms seem like they would be able to much better supply ammunition to their troops than steppe folk, but steppe nobles can expect to have a large number of arrows (one gorytos from a kurgan had hundreds of them!)