Lot of people like making fat characters and while not made for it Virt-A-Mate has it.
It is made mainly for VR users but it also has a desktop mode. Because it was made
For VR you need quite a bulky PC. It is free on patreon but with limited features.
I would like to shine some light on the game because making weight gain scenes is
posible but hard (i tried) and there is so much you can do with the engine.
An example of my favorite model:
I hope you found my post helpful enough.