It had been a strange past few days.

When Twilight Sparkle had been working on a new magic formula in the Royal Canterlot Laboratory, Princess Celestia had happened to check in on her just as Twilight made a miscalculation and the formula exploded on them both. When the smoke had cleared, the two had laughed it off, being only slightly dirtied. Then Celestia's former student grew breasts as large as her head, which prompted them to perhaps take the side effects of the accident a little more seriously.

Over the next day, Celestia had experienced the same breast enhancement, and an even stranger transformation took hold soon after.

Not that Celestia minded much.

The princess of the Sun sat on a terrace, sipping from a dainty white teacup, her long glowing hair undulating around her, caressing her enormous shoulders and arms. Her simple white gown just barely contained the vast globes of firm flesh that were her new breasts, supported by the exquisitely developed muscles of her chest and back. A slight breeze sent a rippling effect over the short white hair that covered her body.

"Ah..." Celestia set her teacup down. Even with her massive muscles, her hands remained relatively small and dainty, and she was graceful enough that she had adjusted to her new strength in no time at all. She had no problem handling herself.

Twilight on the other hand... she thought.

Crash! Celestia winced as something heavy fell over, possibly one of the guards.

"Sorry about that!" came Twilight's voice. "I promise I'll get that door fixed!"

Twilight spilled out onto the terrace, disheveled, her new wings flapping about with all the grace of a chick still learning to fly. Small, skinny Twilight, who had never been very active even for a unicorn, now had more muscle than the strongest of earth ponies and pegasi. Her purple dress was strained tight over her giant breasts, which were heaving most alluringly, and her arms had grown so much that the sleeves had burst. Celestia made a mental note to get something more elastic for Twilight.

"Twilight..."

"Sorry Princess!" Twilight said, standing up, blushing through her purple fur. She avoided Celestia's eyes, her shoulders and wings slumping. Unlike Celestia, Twilight seemed awkward with her hyper-developed figure, and it was like she was trying to hide it. "I keep trying to control my strength..."

Celestia stood up, towering over the new alicorn. "Then I'll just have to help you personally!" she announced, feeling a little nostalgic for having a chance to be Twilight's teacher again, if only for a little while.

Twilight gasped as Celestia took her by the arms, and gently lead her across the terrace. "Small steps, like this!" Celestia instructed. "Follow my lead!"

"Small steps..." She and Twilight moved around together on the terrace. Their huge sizes made it all a bit intimate. Breasts would bump each other as they tried to navigate the small area. Were they dancing? Celestia supposed they were, and it was kind of enjoyable.

"Getting the hang of it?" Celestia asked.

"A little, Princess." Twilight blushed and gave a nervous smile. "I'm really trying to get this reversed for us both."

"Reversed?" Celestia asked. "Please, don't reverse me! I quite enjoy this new figure."

"You do?" Twilight was incredulous.

"Of course. I like to walk out, bosoms thrust forth, standing tall and powerful," Celestia said with a grin.

"You don't feel awkward or clumsy like I do?"

"Of course not! I'm a princess, trained for many—"

Celestia backed into her tea table and tripped over it, knocking her table and chair over, and getting drenched with her tea. Grinning sheepishly, Celestia plucked her teacup from her snout and set it aside with exaggerated daintiness. "As you can see, that teacup is perfectly fine."

Tea soaked her dress, making it cling transparently to her body. Her big nipples were obvious to anypony looking. Twilight gasped and looked away.

"It's all right, Twilight!" Celestia laughed. "Nopony else is here to see us!"

"It just feels indecent, seeing you like that..."

"Oh, Twilight. Do you think I don't have any sexuality? That I would never want to look sexy?"

"It's just kinda weird to think about."

Celestia smirked a little and stretched out her wings, pushing her breasts out and flexing her arms, her massive biceps rising like mountains. "You could never say I'm sexy?" she asked.

Twilight starred, a shocked at the display, but there was something else too, a fascination.

"Princess...Celestia..."

"Hmmm..." The feeling of Twilight's eyes pouring over her was thrilling, intoxicating even. Celestia had not let herself be admired so intimately for a long time. It felt good to cut loose again, and what a naughty thrill it was to parade her charms before her old student!

"...I want to marry you!"

Celestia froze dead in her tracks, her eyes snapping open. Had she really heard that? "...Marry me?"

Twilight buried her face in her hands. "I'm so sorry!" she sobbed. "It just came out! I'll go now!"

She turned to leave, but Celestia grabbed her arm, spinning her around so they were face to face.

"You would really want to marry me?" Celestia asked, astonished. "I was hoping for...well, a little fooling around—I haven't been intimate in centuries, and I am very fond of you..."

"Princess?"

Celestia smiled and nuzzled Twilight's cheek with her snout, their breasts rubbing together and Celestia's fingers tracing her fur on Twilight's mighty bicep. "Even after everything you've been though, I still foolishly underestimate you, Twilight. You're a full mare now, with your own desires and needs."

Getting on her knees, Celestia rested her head to Twilight's bosom and closed her eyes. "There's nopony I'd rather marry than you, my faithful student."

Letting out a happy sob, Twilight embraced the princess, and they held each other as the Sun set.

Author's Note

This story started a bit shakily for me; I sat for a while, wondering what I should be writing. Then I put the first sentence down, and the rest just flowed out. I have wanted to write about a muscular, busty humanoid Celestia for a long time now, and having Twilight in there too makes it even better. I know, not my best effort, but I had fun with it.