Kassaz commissioned this artwork and wrote this short story to go along with it, also available in this anthology:
https://www.fimfiction.net/story/569242/4/tiny-stories-with-huge-pregnant-ponies/royal-brood

The word for a group of alicorns has been lost to time. Most ponies simply call them “royalty” now. In the times of only one alicorn, “princess” alone sufficed. Princess Celestia was personally fond of calling her pregnancies simply her brood, and herself a broodmare, especially when some new member of the nobility had never seen her pregnant up close and was taking a long drink.

Princess Celestia always enjoyed being at her most feminine, when her body took on a shape more closely resembling her sun, on account of her new sons and daughters. Ponies with painting cutie marks throughout history have begged to put her fecund frame to canvas, she could see the many paintings of past pregnancies lining the hallway of the castle in which she waddled, but more recently she’d taken a preference for photography. It was nice to have a memento that took minutes rather than hours to receive, although the objectivity of the device meant a certain loss of character.

She brought a hoof to her mouth as she giggled, and magically grabbed one of her favourite portraits from the wall to hover before her. She appreciated all of the portraits in which her subjects made her the subject, and each pony imparted his or her style, but that in her magical grip always made her giggle most. Her head had been depicted as triangles and her legs as crooked rectangles, which bent over or curved around the simple circle depicting her occupied womb, looking much like her sun—with a slight orange colour to match; her large wings nearly disappeared into the background.

Her magic placed the portrait back and grabbed another. While the temple of her body had played home to many ponies at a time before, she’d never been quite so big as it had depicted. The next portrait she viewed was much more realistic: She lounged on her side, propped up by a pillow, while her belly hung over the side of the seat propped up by many more; it was so realistic, she could see the extra fat on her hips and neck from that particular pregnancy.

Each pregnancy, she told herself she’d keep her figure, and for the five hundredth pregnancy in a row she was wrong.

She looked at a photograph next. Seeing herself portrayed in only black and white worked to emphasize those parts of her that her mane overshadowed. The artist had joked about a more risque pose than any other portrait, which she’d obeyed much to his embarrassment, and much later the embarrassment of her children who’d participated in the pose. Ponies told her the camera added some weight, about a dozen foals’ worth in that photograph, she chuckled. The memories of her many children flooded her mind to join in those flooding her body. Her womb had carried strong legs, wide wingspans, and long horns, but never all three on the same foal. Melancholy crept into her mind next.

A long growl from her black hole of a stomach disrupted the quiet hallway, and her thoughts, prompting movement vaguely resembling a hoof across her smooth white fur. “That’s one nice thing about being gravid: I can eat as much cake as I want!”

-

Princess Celestia frowned at the cracked dish on the ground before her, with a ruined cake underneath it. Foals always made a nice table, until she actually relied on it. A lazy shift, a stretching wing, a quick pop of magic, whatever it had been knocked the cake off-balance and down the front of her belly to the floor. She’d afterwards leaned down to get a closer look—eating it anyway had crossed her mind—and was quite enjoying the sensation of scratching the underside of her barrel on the rug for just a moment. She frowned anyway. It was such a sad fate for a cake: Rather than becoming a part of royalty, given life in her body and through it theirs, it would just end up in the trash where it would slowly rot. It had been a vanilla strawberry cake at that, one of her favourite flavours.

“Well, chocolate is nice too.” The chocolate cake balancing on the other side of her belly started to slide off as well, courtesy of a quake from underneath it she clearly recognized as a wing this time. Her magic caught the plate, and half the cake remained on it afterwards.

Princess Celestia was not eating these cakes, not in front of anypony, anyway.