I managed to find the story. It has been reposted by verucagloomp on DeviantArt, originally by Happyguy, called The Cake.
I believe this story is in the library here but I cannot remember the title.
The basic plot centers around a girl and her friend visiting an aunt's, where they try some chocolate cake. The aunt catches them and urges them to keep eating, until they are completely round, like blimps. Pretty sure they keep repeating the word "Blimp" too.
Any help is greatly appreciated.
Is this is?
https://www.deviantart.com/crashoverrid/art/Say-Babies-439012958