Macro Hyper Rarity (Futa or Plain, Up to you) wearing Twilight and Trixie where Twilight has been turned into a dress with her face showing up on the front of it and Trixie as the panties with her face spread across her rear. Feature some growth anytime they try to use magic.

Ponyville shook as Rarity took another step through downtown, the very ground sinking and cracking beneath her gargantuan hooves. She grinned as she realized she’d reached Carousel Boutique itself. The tip-top of the building came up to just under her chin.

“Oh, I absolutely love seeing the Boutique like this,” she said, her great voice rumbling out over the surroundings. “Like a tiny toy carousel. What do you think?”

She grinned and placed her hands on her hips - or, more accurately, on Twilight’s hips. The dress she was wearing had been among her finest work, made from pure alicorn. She could feel “Twilight” jerk a little bit at the touch of Rarity’s hands and giggled. The entire front of Twilight’s face took up the front of the dress, and it currently did not look happy.

“I think it looks like you should let us go, Rarity!” Twilight said, choking out the words. It was hard to speak when you were a piece of fabric, but somehow, she managed.

Rarity just giggled to herself. “And give up a dress this lovely? I think now.” She slid her two hands up her hips and across her bodice, ravishing the smooth, purple silk. The bulk of the dress was taken up by the front, with the top half stretching far out in front of her chest as it struggled to hold in her more generous “assets.” They’d grown with her, of course, and she would have found it a shame not to put two of her most bountiful gifts on full display - right beneath dear Twilight’s face, of course.

She did a little hop and giggled again when her breasts hopped with her, the two lifting up into the air by a few feet and then smacking down into her belly with the most satisfactory sound she could imagine. She shivered as she felt them jiggling. Just the feel of their weight, of their pull on her chest, was enough to get her wet. Had she been a normal-sized pony, each one would have been the size of a barrel of cider. As it was, with her height of fifty-five feet, each one was a good twenty feet tall, if not more.

Still, though, her bust wasn’t her best attribute. No; that honor went to something new. Rarity grinned as she slid her hands back down Twilight, the dress crinkling slightly beneath her hands - and Twilight groaning (or moaning) as she did - and toward her crotch.

The dress went all the way down to her knees (Twilight was such a conservative dear), but it was what lay underneath the dress that really got Rarity excited. Licking her lips, she moved her hands downward until she felt the beginnings of the great, thick bulge that now dominated her crotch. It protruded obscenely from her hips, and as she slowly rubbed her palms in wide circles across it, it began to grow.

“H-hey!” she heard, from somewhere below and behind her. “Stop that!”

“Stop what?” Rarity said, smirking. Right below her dress, barely covering her thighs and straining to hold in her ass and package, was a teal-colored pair of panties with Trixie’s face stamped on the rear. Now, Twilight had been just a test subject, if you will, but Trixie…Trixie had been a gift. She’d been asking for it, really.

“Th-that!” Trixie choked out, as Rarity’s strokes increased in frequency. Rarity’s smile only widened.

“What?” she asked innocently. “This?” With that, she began to squeeze and stroke her package even harder and faster than before. She felt more and more weight growing between her legs, her arousal heightening until, suddenly, she saw a cocktip poking out from the bottom of her dress. But it didn’t stop there, with Rarity continuing to massage her cock - and, by doing so through her overstretched panties, massaging Trixie’s ass - until it towered above the first two floors of the Boutique and had lifted her dress so high that the enormous twin orbs of seed hanging between her legs - both somehow held by her Trixie panties, but neither actually concealed - became completely visible, along with the entirety of her thick, voluptuous thighs.

Rarity just shivered in pleasure as she looked down at the rest of Ponyville, which by now was staring up at her and her enormous, dripping member with shock, disgust, and undisguised lust. “That?” she said again.

“Yes!” Trixie grunted. “Nff - stop - rubbing - me!”

Rarity raised an eyebrow. “Or what?”

“Or this!”

Twilight reacted instantly. “No! Trixie! Don’t!”

But Trixie was angry enough to ignore her completely. Rarity just smiled and closed her eyes as she felt the magic channeling through her panties - and then smiled even wider as she absorbed it all. Inch by inch, and then foot by foot, she started to grow. Her breasts swelled too, growing rounder and rounder even as her erection reached its full height. By the time she’d hit sixty-five feet, her shaft stood rock-hard, and vertical enough that it’d managed to wedge itself between her two marshmallow-soft breasts. Rarity just moaned, rubbing Twilight as she effectively titfucked herself.

Up and up she went, passing seventy, then eighty feet. Her gigantic, churning balls cast a shadow over the entire downtown, and her bust had gotten so big that it blotted out a good portion of the sky for anypony beneath it. Rarity’s hips just pumped back and forth, at first slowly, then faster and faster, increasing the speed with which her cock thrust up through the neck of her dress.

Finally, her growth petered out at an even hundred feet, and Rarity’s thrusts and libido slowed with it. She opened her eyes, and grinned down at Carousel Boutique, which now barely came up to her crotch.

“An excellent job, Trixie,” she said, as Twilight fumed on her chest. She reached down and gave her erection a lick between her boobs, shivering at the touch of her warm, moist tongue. “Shall we go again?”