As mentioned by @Xarian, here’s a link to a thread discussing the Sims 3 Gro Mod, with relevant links: The Sims 3 Vector mod add-on Gro Virus
Here’s a thread with one persons compilations of fat mods for the Sims 4, they should be easy to find individually in other threads in the General Games category: The Big Fat Sims 4 ModPack
And finally, a whole thread of more general mods featuring weight gain and extra sliders for 4: Sims 4 mods