It's just a prank sis.

Luna was sitting in her room playing some video games with her sister.

"Damn it!"

Celestia threw her controller in frustration at having lost once again to her little sister.

"You have to be cheating!"

"No, I'm not, I'm just better than you."

Luna giggled as her sister picked up her controller for a rematch. Celestia knew something was wrong, Luna was good but not THIS good. They started the next round, but this time Celestia kept a closer eye on Luna. It wasn't long until she saw her trying to sneak in some cheat codes.

"Luna! I knew you had to be cheating!"

"Oh fine, but really I don't need them, I just wanted to see how'd you react to getting COMPLETLY owned! Though, it might help you if you ate fewer sweets, particularly cake. So many rots the brain, not mention it's making your butt huge!"

Luna had just begun to laugh before giving out a yelp as Celestia jumped on top of her, pinning her down as her horn begin to glow.

"Ce-Celestia?!"

Celestia had a devilish smirk on her face as she lowered her horn to Luna's chest.

"Don't worry, I just want to see how you'll react."

Touching her horn to Luna her magic quickly soaked in through her fur before disappearing altogether. With a mischievous laugh Celestia quickly hopped off Luna and ran out of the room before Luna could recover.

"HEY, what do you do to me?!"

Her only reply was her sister's laughter echoing from down the hallway. Before she had a chance to give chase she began to feel something happening to her body. Looking down she nearly screamed. Before her eyes her body was blowing up! Her stomach began to distend with fat, reaching a full foot further to the ground than it had before, her flank following behind to double itself in size. Her legs went from thin rods to lumpy logs, her neck sagging a bit with an extra layer of fluff. Lastly was her face, getting a bit rounder with the hints of a double chin forming.

Overall the now pudgier Princess had to have gained at least a 150 pounds!

"CELESTIA!!!"