Muscle growh with Rarity and Rainbow Dash, please. Rarity finds out she builds muscle instantly by workibg out. She loves it, but Dash gets more and more jealous as she gets bigger.

Rainbow Dash wiped the sweat off her forehead. “Whew.” She let her hands rest on her hips as she straightened up. Alright. Now that her stretch routine was over, it was time to -

“Yoo-hoo! Rainbow Dash! Am I doing this right?”

Dash took a second to swig down a gulp of water before jogging over. “What’s up, Rares?”

Rarity was at the rowing machine. She had her legs delicately crossed over one another, and her fingers perfectly positioned with her nails clearly away from the pull-bar. “I’ve been rowing for a full minute, but it just feels odd.”

“That’s cuz your technique’s all off,” Dash said, bending over. “See? You’ve gotta hold out your legs like this and then push off, hard. And you can’t hold your fingers away like that; you’ve got to grip the bar hard and hold it to your chest.”

“But - my manicure!”

“No buts.” Dash shook her head. “You wanted to get buff, right?”

“Get buff! I - nothing of the sort!” Rarity said. “I merely wanted to get more in-shape. But…you say these techniques will improve my form?”

“Gotta do it right if you want to get in shape.”

“Very well,” Rarity said, cautiously. “Thank you, Rainbow Dash.”

As Dash walked away, she failed entirely to notice the small bulges on Rarity’s arms and legs where previously slender limbs had been. It wasn’t a notable gain - two or three pounds of muscle at most - but it was a change. Rarity, for her part, failed to notice them entirely.

As she resumed her workout, though, some part of her mind registered that when she let her muscles relax, they stayed the same size as when they’d been curled. She just kept curling, the sweat on her forehead coming down less and less as her muscles grew and grew.

Soon, the training bra across her chest had grown much tighter, her chest having swelled with muscle as much as her abs had tightened and her hips firmed. Rarity nearly moaned as she made another pull - no proper lady would have done such a thing in public, of course, but it just felt so incredible to have that tightness stretching across what felt like her entire body. She imagined that this was what after-workout soreness felt like, but failed to realize that her workout wasn’t getting harder.

If anything, that same corner of her mind realized as she stood up from the bicep curling machine, it was getting easier - and she was getting bigger.

Just under an hour later, Dash was at the front desk, doing a final towel-off to wipe away any sweat still on her face. She was panting, but just barely. She’d had a good workout. She wondered how Rarity had done.

“Rainbow Dash?” Something thudded on the ground behind her, and she turned around with a smirk, tossing the towel over her shoulder.

“So,” Dash said, “how was your first day at - “

Then she saw Rarity’s straining shirt, saw the muscles rippling beneath the stretched, nearly-ripping sleeves. She saw the thick, corded muscle running down Rarity’s legs, the bulging spheres of Rarity’s tight flank pushing through her track pants. She saw Rarity’s bust, bulging almost to Fluttershy’s size…and then she realized that she was looking Rarity directly in the eye, when she normally should’ve been two or three inches taller.

“What the hay?” Dash nearly jumped. As it was, it felt like every hair on her body had stood on end. Rarity, the prissy little dressmaker who had been barely willing to risk breaking a nail now had Dash’s own physique - if not a little thicker physique, something deep in her gut noted enviously. “Rares, don’t tell me you took some bodybuilder’s potion off ‘em in the locker room!”

“What, this?” Rarity glanced down at her body, at the straining clothes, at the lean, hard muscle. “I thought that this was what one did when working out.”

“Not like that!” Dash said, sputtering. “Not that quickly! Holy crud, Rares. I - I…”

She was going to get to the bottom of this.