Soarin' gets jealous/angry of Mac being the "stud" in town, uses an amulet he stole to steal some of Mac's "features" (Muscle/endowment/size theft, domination)

Big Mac looked down at Soarin’, quirked an eyebrow, and drawled: “What’re you gonna do with that little doohickey?”

Soarin’ just sneered up at him. “I’m going to even things out.” He clutched the amulet a little more tightly in his hoof.

Big Mac just snorted - and then choked as a bright golden haze bled through the air around him. He felt it seeping through his coat, into his body, and somehow even deeper than that.

“Oh, yeah,” Soarin’ groaned. He rolled his neck as the same aura, only brighter, flickered around him. “That feels good.”

Big Mac grunted. Suddenly, his whole body felt a whole lot heavier. Looking down at his chest, he could see the muscle there falling away. His legs wobbled beneath him, and he realized that they were getting thinner, weaker, and less muscular by the second.

He collapsed back onto his haunches, wincing, just as Soarin’ flexed his. “There we go,” Soarin’ said. His legs were bulking up, slowly but surely, and his chest and rear were already thicker with long stretches of thick, corded muscle. He exhaled harshly, grinning to himself. Mac could swear he saw two spurts of steam rush out of the pegasus’ nostrils.

It wasn’t long before Mac found himself staring directly into Soarin’s eyes - not, as he was used to, looking down into them, but looking forward, from an even height. Big Mac groaned as the glow around his body flared. He felt himself shrink a few inches, even as Soarin’ grew even taller.

“Now that’s more like it!” Soarin’ smirked down at him. By now, they’d completely reversed sizes - Soarin’ was sure to be the largest stallion in all of Ponyville, towering above most of the other ponies for miles around. And Big Mac was…average. Weaker, maybe, than the average stallion, but not by much. But as he watched Soarin’ flex and growl under his breath, his growing muscles bulging and throbbing even as Big Mac’s faded, he had a feeling that the other stallion wasn’t going to stop there.

Soarin’ just groaned and flexed as the amulet fed him more and more size and strength. Keep it coming, his mind demanded. More. More. He felt a powerful stirring between his legs, and glanced down between them. A two-foot horsecock, its head flared almost too wide to fit comfortably in any pony’s muzzle, thumped against his stomach.

It stretched all the way from his crotch to just a few inches away from his chest. A heavy, orange-sized pair of balls dangled between his legs. As he watched, each one bloated up a little more with cum. He could feel his seed sloshing around inside, their volume growing by the second.

Before Big Mac could react, Soarin’ had lunged forward and pinned him to the ground. “Let’s take a look at your little prick,” Soarin’ said, sneering, and before Macintosh knew it his fully erect cock had plopped out of its sheath and stretched over his belly. It didn’t even reach five inches.

Mac swallowed. The mares in town had once called him “Big Mac” for more reasons than one. Now…

“Not so big now, are you?” Soarin’ gloated. Big Mac gulped. Already, he could see Soarin’s enormous cock growing even longer, passing two-and-a-half-feet even as his own shrunk down to two inches, each of his balls just barely the size of a chestnut. Meanwhile, Soarin’s massive body pressed down on him, his gigantic cock oozing precum onto Big Mac’s tiny, shrunken chest. Mac couldn’t have been larger than a filly by now; meanwhile, Soarin’ could have been even taller than Celestia herself.

Soarin’ grinned and pressed down even harder. His gigantic, throbbing cock served as the perfect-sized weight to keep Mac pressed to his back. He didn’t even need a single one of his enormous, muscled legs to keep Mac down. Mac just grunted, struggling to breathe as he shrunk down another inch, and Soarin’ added another.

Finally, the golden haze disappeared, leaving behind a gigantic, horny Soarin’ - and a tiny, weak, tiny-cocked Big Mac. Mac just stared in shock and sorrow at his inch-long cock, his tiny, laughable legs.

And Soarin’ just grinned down at him. “Much better.”