This is a long shot, but I have been looking for this story for quite a while.
it was on Fantasy Feeder years and years back. I believe it was called Cafeteria Girls, but I have had terrible luck with web archive.
Here is the only excerpt from the story I have.
"How can you eat so much?" Charlotte asked Lucy.
"Probably because I'm always hungry." Lucy explained.
"Won't you get fat though?" Charlotte asked.
"I doubt it; I always eat and eat but only gain like, 2 pounds. Anyway, I kind of need to gain some weight." Lucy replied.
"You’re lucky then, unlike me." Charlotte said whilst showing her belly to Lucy.
"What’s wrong with that?" Lucy asked.
"Well look how fat it is!" Charlotte said whilst grabbing a large fold of fat in her hand.
"It's not fat! Anyway, it’s better than being a twig." Lucy said.
"Really? Don't be silly." Charlotte said in disbelief.
"No really, it actually looks cute." Lucy replied.
"Naw, thanks Lucy." Charlotte said whilst smiling.
