An attempt to prank Celestia with Poison Joke goes wrong when in private she turns into a giant cake and is found by Luna. Weight gain and vore.

“Oh, my.” Luna eyed the big cake with no small amount of interest. “What’s this?”

Silently, upon the table, Celestia screamed. Or would have, had she possessed a mouth. As a cake, she didn’t have much in the way of mental or verbal faculties. As it was, she was completely unable to do much of anything when Luna approached the table, her slightly chubby belly swaying below her as she eyed her “sister” with an obvious hunger.

Luna glanced over her shoulder. Surely a cake this magnificent couldn’t have been left all alone in the middle of the kitchens? It was a full eight-layer cake, angel food cake with vanilla frosting, and absolutely covered in powdered sugar, caramel, and strawberries. Her stomach was growling just from the scent. She gave another sniff and unconsciously slurped up a bit of drool. Surely nopony would notice if she took just a little bite.

The first bite was heaven. She chewed it slowly, thoughtfully, savoring every crumb of the sweet morsel that passed through her lips. When she swallowed, though, she didn’t feel satisfied. She didn’t feel full. No: She wanted more.

“Just another bite,” she told herself. The second bite tasted even better than the first. What was left of Celestia watched helplessly as her younger sister munched on her top layer, smacking her lips happily as her chubby cheeks bulged with cake.

If a second bite, Luna reasoned, why not a third? It’s not like three was that much greater than two. Before she realized, she’d taken another half-dozen bites. By then, any notion of self-control had flown out of the window.

Luna moaned as she gulped down a full slice of cake. The frosting smeared across her face, and her throat bulged a bit around the dessert as it went down. She’d been enjoying her new cushy life in Canterlot, as evidenced by her new waistline and bulging flanks. All prior pig-out sessions, she’d managed to write off as “the appetite of a princess.” But this time was different.

As she gulped down yet another slice, her belly started to round out even more. Maybe it was her sister’s magic, contained within the cake. Maybe it was the aftereffects of the Poison Joke, now becoming something more like a Poison Bloat. Or maybe Luna’s metabolism was just going into overdrive, working hard to convert every calorie of her delicious treat into pudgy, glorious fat.

So Luna ate and ate, her belly dropping lower and lower as she gulped her sister down. She filled herself up, but she didn’t feel full. If anything, she only felt hungrier. The fat flowing out to fill her limbs and belly only seemed to increase her appetite to previously unseen heights.

Her flanks rolled and swelled, rolls of fat drooping down over her haunches. Soon, each one of her Cutie Marks would have been the size of a normal pony’s head. Her belly had just passed her knees. She’d finished growing a second chin. It jiggled whenever she chewed, and she was well on her way to starting a third.

Now heavy enough to match two - or even three - ponies in weight, Luna let herself fall back onto her flanks. Her quivering rump smacked onto the floor with a mighty force, but she felt none of it. The generous padding on her ass saw to that. No; instead, she focused on levitating the second half of the cake before her, fully focused on scarfing the rest of it down as fast as she could.

On and on she ate, and on and on she grew. The cake, which had once been as big as a mare, was now just barely one-third that size. Meanwhile, Luna looked to have nearly tripled hers. When lean, the princess had struck a mean of one-hundred and seventy pounds; after her weeks of overindulgences, she’d weighed in at a pudgy two hundred and twenty. Now, she looked to have reached a solid five hundred pounds of mass, and she still looked ready to grow.

She ate and ate and ate, unknowingly consuming the last of her sister in her greed and gluttony. Finally, she reached the last slice of the cake. Luna eyed it lazily, greedily - and swallowed it in a single, gluttonous bite. Her following belch actually shook the pans hanging from the ceiling.

Luna patted her great, powerful belly, loving the feeling of the taut skin beneath her hooves. “Magnificent,” she rumbled. “Delectable. And tasty.” She amused herself by playing with her fat for a minute or so. It was so soft, so pudgy. Rolls of flab cascaded over her body, shifting and massaging her every time she moved.

And then her stomach growled again. Luna grinned, her chubby cheeks spreading wide as she listened to its plea. “Still hungry,” she mused. “Mayhaps I’ll find something more in the pantry.”

It took a lot of work - and magic - to heave her royal bulk to her hooves, but somehow Luna managed it. Then, with her third chin slapping against her neck and her belly dragging along the floor, she drew herself up and made her way out the door. There was yet food to be consumed this night.

On the counter, an empty platter sat in forlorn silence. Not a bit of poor Celly was left.

Luna hadn’t even left a crumb.