Super Mario Bros defies many different forms of physics. Assuming his round belly carries some weight, I would be nice to give him a weight around 200 pounds. Since he can jump over 5 times his body weight, his legs would need to lift at least a tonne to come close to the approximate strength to perform such a feat. In addition, kicking the koopa shells would create an equal and opposite force back onto his feet. He would not be that agile if he were wearing steel-toed boots, so it should hurt a regular human. Then again, can you imagine how boring that would be?
Thanks to the beauty of the internet, here is a small physics lecture from mario himself!