One of the biggest misconceptions about shock combat is that the combined weight of horse and rider is directly translated to the lance - As if somehow the horse, rider, and lance were one rigid mass. In fact, they may move down the field as one, but at the moment of impact, they react as separate units.
In reality the rider's body acts as a shock absorber, or buffer, between the lance and horse. It cannot be stressed enough that the rider's own strength and weight are the key to translating the mass of the horse into the force of impact. Although the size of the medieval warhorse gradually increased over time, the effective size of the lance and horse interface (the rider) did not.
It is the resilience of that interface between lance and horse that determines the amount of energy put into the impact. Thus the amount of the horse's mass that could be effectively put behind the lance was directly determined by the rider's size and strength. In other words, a larger, stronger horse does not mean a harder hit, while a larger, stronger rider usually does. In fact, in a joust I am more concerned about a large rider on a small horse, than a small rider on a large horse. Velocity can increase or diminish the relative force of the impact. This is true both in terms of hitting and being hit.