Originally posted by The Scenario
Shrapnel and spikes plus both forces.Edit: Though it is a relatively low showing for Bowser.
Shrapnel and spikes don't contribute to the forces, and if the shrapnel was moving fast enough to damage Bowser then it would have traveled far enough to hurt Mario as well; it doesn't.
Jesus, guys. We all know that the answer is that the reason why Mario is shown lifting a castle but can't toss Bowser farther than 50 feet is because it's a cartoon, and we all know that the reason why Mario has the power in his legs to move fast enough to dodge lasers but needs a water jetpack to jump higher than 20 feet, is because it's a cartoon, and it's not designed to make sense, it's designed to be fun and goofy.
edit- And as far as showings go, it's not just a "showing" because it happened that way in every fight throughout the game, therefore it has enough consistency to be considered a character trait. So what makes that a low showing for Mario and Bowser, instead of the castle lifting just being cartoony PIS? Both feats are shown often enough in the series to be considered traits of the character, but you want to pick one over the other.