Fat representation is not as trendy as lbgtq+ and racial representation and at the end of the day is all about money. Does the cost of adding fat bodies to the game will make enough profit? If not, there's no point for them to add it. It is what it is.
Larian proved at least to an extent, that to them it is not only/always about money. People don't even make the DLCs. I can totally see them adding it just to make the RP aspects of the game better.
