This story was inspired by this picture: http://www.furaffinity.net/view/5876709/
I assume, I went a bit wild and it is a short story. I do wish for you to enjoy.
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After a thousand years, Luna had finally returned home. Her room had been held clean and tidy through all the time, much to her suprise. It did seem her sister cared greatly for her. However, she did wonder, the bed.. had it been so great and wide as she had left? Regardless, she jumped into the matress, feeling how soft and comfy it was. Rolling about, she finally halted on her side, letting her memories drift off to the evening they had.

They had celebrated a great festival, just for her, had gorged on food and drink to give the night Princess what she deserved. A party, pardon you, not the food. But still, as she patted her taught and rounded middle, which replied with a gentle wiggle. It was merely a small buldge that she would be rid off soon, luckily.
However, Celestia had mentioned to meet her in the chambers after the party and still hadn‘t shown up yet. Time went by, but Luna was to lazy and full to move even an inch. Slowly, her eyes closed, letting her slip into a slumber, only to then awake to the gentle voice of her sister.

“Luna? I am sorry I couldn’t make it here earlier.” the white mare set herself aside from her sister, gently stroking a feather of her wing over the filled middle of the blue mare.

Luna smiled gently at her sister for the gesture. “You wanted to suprise me, you said.”

“Yes, that I did. Do you remember the evenings that we shared, before your banishment?”

Luna sat up, but was gently pushed back down by Celestia, to lie on her back. After a moment of confusion, the little sister looked up to the brillant white mare. “I remember. You would feed me little treats, until I was so full, that I fell asleep.”

Celestia nodded with a smile. “Exactly. And today, I would like to perform this ritual again, as a little, welcome home gift.”

“But I am already full...” she was silenced by Celestias gracefull wing tip, set to her lips.

“Shh, dear sister. I have prepared everything. Relax and do not worry this night.”

It took a moment for the little alicorn to calm down and simply rest. Her head was prepped up by a pillow and her sister instructed her to not move, but just to eat. As she once again wanted to plea not to eat, as her stomach was full already, Celestia moved her horn on to her belly, letting it glow warmly. Her eyes locked on the face of her sister.

“Does it feel better now?”

Luna widened her eyes. Not only had the pressure and her fullness go, it was replaced by a void. Suddenly, she felt like she was halfway starving. It took her several moments to get along with the feeling that contradicted her vision. It still looked full, but she felt so famished!

“What did you do?”

“I freed you from her physical boundaries. Now, enjoy..”

A first little cake found its way to Lunas lips, chunky, with chocolate and marzipan. The smell and the emptyness in her stomach was enough to make her force it into her mouth and swallow, without much care of chewing. She had her front hooves feeling over her stomach, that made a noticable growth within her grasp. Yet, the burning desire to eat and the fact that her Sister would keep offering food made her unable to stop filling.

Fruits, vegetables and plenty of sweets were followed by bigger dishes, that the kitchen staff brought along. The smells were so delicious, the portions so massive, that her inner conflict grew along with her.

“Do not forget to drink!” Celestia teased her, slipping a bottle of milk to her poor sisters lips, making her drink it in a single setting. The buldge that she once had, was beginning to stretch out more and more, so that Luna began to have problems seeing over it. She could feel the increasing weight pushing her down into the matress and hear her skin moan and creak as she was starting to be filled by the next meal again.

Her mind raced, as regardless of the sounds her body made, she still was unable to stop eating. She manged to splurt words between the bites, but every bidding to stop, every plea to not make her eat more, was simply not heard by her Sister. Or it was blessed, with a bigger bite.

“Calm down, Luna.” the celestial mare said friendly and gentle, carrassing the fragile and growing orb of a stomach. “You will be fine. Don’t you trust me?”

Tears rolled down her cheeks. Luna wasn’t sure if she could trust her sister at this point. Had she done so much wrong, that she wanted to make her burst in what used to be a warm, funny family event of eating a few treats? Was this her punishment?

Dish after dish was eaten, Celesta staying calm and sporting a smile, as she kept pushing new food into her sisters hungry mouth. Only her hoof poked the widening belly sometimes, as if to test how near she was to bursting. And with each poke, Luna noticed that the stomach gave in, less and less.

Her breathing became heavy, as the belly had grown big enough, to block her vision entirely and start to droop at the sides, laying on her legs. It appeared to her, that movement was now fully out of question and all she could do is feel the pressure pile higher and higher, as well as her midriff grow. She whined at the fact that her only sight was the pinkish skin beginning to show through her dark blue fur, indicating that she might really pop in the end, as the food seemingly simply did not stop.

Suddenly, Celestia began to sing gently.

“Once was a pony, of a deep and dark hue,
she said to her sister ‘I’ll be just like you!’
But then she said something, that she might regret,
and something her sister, will never forget.

‘Look at the moon, as it lights the night sky!
Oh how I wish, that moon there was I!’
Though someday that greed, to be noticed, outgrew,
the hunger, to be like the moon that they knew.”

Luna heard a painfull creak from her belly, as she swallowed down the next bite, her legs being more and more pushed down. She knew she would burst... yet her sister went on.

“Much troubles she faced, but nopony saw,
All that she wanted, was those ponies in awe.
They should bow deeply, before her brilliant display.
And none should be able, to turn their gaze away.

So back in her place, the little one stands.
Her fate however, lies in other hands.
Her sister will now, make sure that she stays,
Not only today, but till the end of her days.”

The mare of the night whimpered, more food wandering in to her, her stomach crying out for it to finally stop, or for her to at least burst....

“And noticed she will be, that she will see soon.
Being greater and bigger, as her brightest full moon.
Because in the end, that was her wish.
So sweet little Luna, eat up your next dish.”

Celestias worked her magic on her round sister, before shoving in more food into her mouth. Luna couldn’t even tell what it was by now, to busy with sobbing and trying to gasp for air. That little thing she said back then, that would be her fate? She was a filly back then! Just had earned her cutie mark! And she knew, ponies loved the moon, forgetting though, that she rose it every night. She was jealous and naive back then!
However she then noticed, her stomach was beginning to grow again after the spell, pressing out painfully and quite unaturally round. New dishes arrived and it seemed like there was no end in sight just yet. Celestia did not really...

“And see dearest sister, your wish will come true.
The moon will be adored, as the moon will be you!
The bigger, the better, and let you be told,
That I will make sure, you grow as much as your body can hold....”

She ended the song with a loving smile, just adding. “Sometimes ponies have to do something they don’t like, to achieve greatness. And since I know it is your dream to be noticed and as briliant as your moon, I will gladly help you on the way, if you like it or not.”
A sweet slice of pie slipped into Lunas mouth, as she just helplessly looked at her sister.

“Do not worry Luna. This is a mere start.”