What about a muscle growth story where Applejack has an armwrestling contest with an overconfident Big Mac? The winner of each round gets a substantal bonus to all physical attributes. Big Mac loses the first round by accident and it all snowballs from there.

“You ready for this?” Applejack asked, smirking. She clasped Big Mac’s hand in hers, real tight.

“Eeyup.” Big Mac grinned back. There was only one word for the expression on his face, Applejack noted with a frown: smug.

Well, she wasn’t about to lose, she told herself. One round. That’s all she needed. “On my mark,” she said. “One.”

Big Mac’s arm tensed. Applejack eyed his bulging biceps warily.

“Two,” she said, and readied her own arm. “ - Three!”

Applejack’s arm suddenly felt like it was pushing against a train. It took every ounce of her will not to let it crash to the table instantly. As it was, Big Mac was almost easily able to force her arm down a few inches - not all the way to the table, but maybe a good third. She grit her teeth and pushed harder. She wasn’t about to give in now. One round.

“Give up?” Mac drawled. Applejack eyed the bulging muscles on his chest, his melon-sized biceps.

“Not a chance,” she snarled. Mac grinned, readied his grip, and -

“Yo, Mac!”

That was her window. For just a second, Mac’s grasp weakened - and Applejack reacted without a second of hesitation. Mac’s fist crashed to the table.

Big Mac looked down at his downed fist with shock and betrayal. Meanwhile, Dash finished jogging up to the table. “Hey guys,” she said. “What’s - “

“Whoa nelly!” Applejack closed her eyes and bit her lip, already feeling the magic take over. She’d been wary about testing out Twi’s new spell, but once Mac had suggested trying it out under certain conditions, she’d been rarin’ to go. And from the feeling burning in her gut right now, she’d been right to do it.

“Mmff,” she moaned, feeling her body bulge and swell. She tensed up an arm, watching the bicep flex, and grinned when she relaxed it and saw the muscle stay the same size. “Come to mama.”

“Y’can’t do that,” Mac said. His eyes were wide. “Y’all can’t win like that.”

“I already have,” Applejack said. She let out another groan of pleasure as her chest swelled, her breasts pushing outward until the buttons of her shirt were straining to stay together. Each one would’ve easily been matched a small melon in size, and she loved the way the fabric rubbed against her chest and arms as she grew.

She reached up in the air and stretched. She could feel her hands staying level in the air even after she stopped, could feel her triceps and core staying hard even after she relaxed them. She could feel her pants tightening around her legs, each inch of her thick, sculpted thighs bulging through the denim.

At last, the growth left her. She lurched forward, her elbow hitting the table with a crash. Every inch of her clothing had been stretched to its limit, her bulging muscles bigger and stronger than on any pony she’d ever seen. Applejack smirked, looking a few inches down toward her “big brother.”

“So, Little Macintosh,” she said, her smirk widening. “How’s about round two?”