Kassaz commissioned this artwork and wrote this short story to go along with it, also available in this anthology:
https://www.fimfiction.net/story/569242/5/tiny-stories-with-huge-pregnant-ponies/full-moon

Princess Luna remembered the word for a group of alicorns, but thought it funnier to tease her sister over it. She opened the doors to her bedchambers and closed her eyes as she took a deep breath. The castle was warm, and she could breathe, both unlike the moon. She would never again take a night in the moonlight, or a day in the sun, for granted. To Princess Luna, her return to Equestria had been more than a return to society and her sister’s side—although she greatly loved both—it was also a return to fulfilment. For one thousand years she had been empty, and she would make up for it, in all ways.

It was appropriate for her bedchambers to have a double doorway, given her gravid width. Her sister had kept the room set aside for her ever since the castle had been built, albeit without her lunar cutie mark that was now emblazoned on the doors. The doorway had been sized against her sister’s widest and then some, but Princess Luna was determined that she could force the castle staff to widen it sometime in the next few decades; the moon was a symbol of fertility, and she couldn’t disappoint the citizenry.

She plucked an apple from a basket with her magical grip and took a bite. Juice ran down her face, and she shoved the rest of the apple in her mouth—stem and core—before walking across the room. Fruit was nice, but she saw an obelisk of sorts standing and waiting for her across the room, and remembered there were greater hungers. Her sister had cultivated a certain taste, and had been more than happy to share what was available. “How long have you been waiting for me, handsome? Let’s get you inside me.” Princess Luna sashayed across the room, swinging far more than her hips the longer she kept going; her powerful wings spread and hugged her sides, failing to keep her gut in check. She nearly threw herself off-balance, but tried not to let it slow her down; her stepping became only slightly uneven near the end to weaken her wobbling. Closing her eyes, she ran her tongue from the base to halfway up the length, enjoying the taste.

The cake was unmoving as she swallowed her first mouthful of frosting. She nearly imitated her sister and started eating it right then and there, but even her royal horseshoes couldn’t keep all soreness from her hooves with the many extra lives they supported. “Let me get on the bed first.” The table on which the cake stood was at the center of her room, though still too close to her bed’s end for her to walk around easily, and she preferred to shuffle her legs slowly and rotate towards the bed rather than to make an arc. She let a small giggle escape her, her enormity and limited movement naught to her but a feature of her fullness. She thought for a moment how she wanted to get in bed this night.

Her wings flapped to bring her near the ceiling; she needed but to slow her wings little to lower herself and land on the bed. Her horn glowed and encased her right hemisphere to press in gently and slowly; once she felt enough on her left side, Princess Luna allowed herself to fall over onto the mattress and many pillows. Her available wing flapped gently to prevent her pregnancy from bringing her to her back as it settled, but she still needed to push it back into place with her magic as well. Now she felt the life within expressing displeasure with its carrier, and she felt nothing but love as she grinned at her personal moon. She was not only full, but also never alone; that she would become fuller delighted her. Her magic grabbed the cake cutter and brought a wedge before her face. “We would normally not allow you to have cake for dinner, but we can make an exception here.” Just as easily as the apple, the entire slice of cake fit in her mouth, and then her throat.

-

Princess Luna brought the platter with the final slice before her, eyes closed to better let her focus on its flavour and texture. Encapsulating magic put every moist crumb in her mouth, and then every crumb left on the plate, but she licked it for good measure and exhaled in bliss afterwards.

She looked back at herself while drinking some manner of carbonated beverage, unable to see any difference in her form; it certainly felt like she had a cake taller than a pony in her stomach. She poked in deeply with a hoof, only to be poked back by one much smaller. This started a game of back-and-forth that ended when she belched after her stomach was assaulted. After the surprise dissipated, she laughed.

Stomach silent, there were other hungers, she remembered. She yelled for one of her guard, who ran inside and saluted. “Guard, we have a heavy need of you.”

He lowered his leg, with another beginning to drop at her tail draped over the edge of the bed. “Yes, princess.”

She set her head down. “Bring us another cake, posthaste, some flavour we’ve yet to try! The castle cooks will know.”

“Y-Yes, princess.”