The main difference is not just weight, it is the twisting force, or torque. the bikes ride much farther out away from the bumper than the tire, thus greatly increasing the torque. Try this, push down hard on your bike rack just above where it us mounted. Then push down just as hard at the farthest end of the rack. I'd bet you can see your bumper twist. Now imagine this twisting happening a hundred times a minute as you cruise down the road --- get the picture? And yes, this does happen, always.