Power is something Bowser holds dearly.
Bowser had discovered that the only way to defeat Mario was to be much more powerful than that annoying plumer. Figuring one of those those ways was to consume others for their power, the best place to start was from his own castle -- why not start in your own backyard?
"Alright! Eating's one of my favorite things anyway. Let's do this!" he said.
One by one, he downed goombas, koopatroopas bombombs, magikoopas -- savoring each of their wonderous flavors upon his thick, drooling tongue. He felt each one of his minions squirm down his throat -- down to Bowser's smelly stomach. Bowser rubbed over his belly, with a hearty deep belch.
"Now this is something I can get used to. Why haven't I tried this before? I'll beat Mario at no time at this rate!" He said with a chuckle as he pat his gut.

Soon enough, Bowser ate and ate, and he grew and grew. Bowser would've consumed enough that the weight of his belly forced gravity to pull the enormous beast to the ground. He grew so much that he could barely move, let alone eat more. Bowser could only sit there in his sloshy self, just watching his stomach gurgle and slosh from the minions inside.

A goomba passed by in the same grand hall as Bowser. Along the way, he glanced to notice Bowser and gasped in suprised.
"S-Sir! What have you done? Where is everyone?" the goomba quentioned.
"Ahhhh... good timing! You're going into my belly, minion. I need to defeat Mario and--" He was cut off by the need to let out a hearty belch into the goomba's face, shoving over the little goomba.
"Ewww.... sir, our forces.."

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Wanted to draw something for my burr,  kewlkwix. (He loves Bowser a freck ton) Enjoy! :3