Fat bodies would be cool, but sadly I can't remember seeing a good adaptation in video games - talking here about cases with player equipment not extra models. Usually they tend to be just slightly wider muscular bodies (SWTOR for example - but they have much simpler models and lower graphics quality to begin with - no where near the graphical fidelity of Larian's BG3). Problem is that the larger volumes tend to cause stretching on textures to a larger degree than just when you add muscles (might not be too bad on cloth, though folds usually look off since they would behave differently, but once your chainmail starts getting stretched it looks bad). More importantly those volumes tend to cause a lot more clipping and are harder to rig and still look good (primarily the stomach area) - which requires custom solution which again ain't something that is usually doable for player characters due to the higher amount of animations that they tend to have. Going from slim to big tough guy is usually much easier to solve since movement isn't that different - you might just want to add custom animations, but even with standard ones its usually not causing too many issues.
So while I would love to have the option I understand Larian not prioritizing it as it comes with a lot of work for even just a decent result (which often isn't good enough for player expectations in AAA-games). Also with all the races and sizes I think Larian has already had its fair share of these problems to begin with. I can imagine rigging and animation teams start sobbing if you ask them to add one more bodytype for all the options XD
