Fluttershy remained silent, but she couldn’t fight the smile steadily creeping over her lips. She knew that Rarity had already lost. If the unicorn had bolted for the door the instant she had secured the amulet, maybe she would have had a chance, but now, she was completely at Fluttershy’s mercy.

Fortunately, Fluttershy reflected, she had no mercy.

“Help me,” Rarity groaned, spreading her stance even wider. “I can’t... I don’t understand! Why is this happening?!”

Gritting her teeth and squeezing her eyes shut, Rarity made a desperate effort to remain on her hooves. Her puffy legs trembled, but her body continued to fill out with every passing second. Her haunches were simply massive at that point, the base of Rarity’s tail undoubtedly lost between them. Her bloated sides threatened to obscure Fluttershy’s view of her flanks. The pegasus noticed that her cutie mark had been stretched out as well.

Pleased with the display, Fluttershy idly wondered if Rarity’s belly would reach the floor before her legs gave out.

“Please, Fluttershy!” she whined. “Go get help! Somepony has to know what’s wrong with me!”

Full Size

(I sure as hell didn't make the background)

(PS this is my least favorite one)