Two weight gain text adventures from the same site - I found the first one linked to on deviantart, and then I decided to try to find if I could find anything else along the same lines. I'm sure this site has some other transformation-themed material, but I searched for "weight gain" because that was what I felt like. Anyway, both have some adult content:
Anna's Wedding : Having already gained some unwanted weight, a bride-to-be tries to resist the temptations of food on her wedding day.
Becky's Binge. In contrast to the above, here you're playing as a young woman who wants to pig out as much as possible without her parents catching her doing it. The one complaint I have with this one is, also unlike Anna's Wedding, there's not a map - if you're not acclimated to text adventures and remembering if a location you need to go to is north or south, you might find yourself occasionally going in circles and getting lost like I did.