Bobert, if the weight of one wheel of a car were lowered onto one's foot, I shouldn't think it would hurt as much. A moving car adds the weight of the 'push' to the psi transferred to the foot, I imagine. Can anyone tell me the physics of this?

There's another reason a large hoof doesn't hurt as much- because it is big, part of it is going to be on the ground and thus it will bear some of the horse's weight. (Think of a plate-sized hoof; many of them are.)