Mass has something to do with it, sure. But when you fall two feet and hit the planet Earth at ~6 m.p.h., it's the same as the Earth moving up and smacking you at the same speed. It's because of the relatively enormous mass of the Earth, or the truck, that the person comes to a fast stop. The reasons the truck hurts so much is (a) it's metal, so there's no give, (b) you're liable to get run over, which will hurt a lot, and (c) you're not generally prepared to cushion an impact in that direction. But seriously, as long as you don't get dragged underneath (if, say, you're knocked clear into a bush), the truck impact really won't hurt as much as you might think. No more than a slip on a patch of ice, and possibly less.