Got some new weight gain mechanics in this update. Now you can feed her any of the foods in the buffet level to make her grow ginormous! But hauling all that fat around consumes a lot of calories. The more she waddles, the more she shrinks until she is back to the default blob size. It takes a long time to work off all that blubber (something like 10 mins of waddling) so as long as you keep her well fed, she will stay at maximum blobbiness.
There's still more I want to do with this, but it's a good start. I'll probably make it so different foods have different growth values and of course I want to put burps in there too. I also want some kind of feeding to be possible on every level, not just the buffet.
A couple other notes. Her weight will not persist between level switches or body shape transformations. There's some issues I have to solve to make that work. Also you can feed her when she's skinny, but nothing happens right now. I want to develop the transition between skinny and blob, but that will take a lot more R&D to get working. In the meantime, I hope you enjoy!