Goddammit, I know this answer to this, at least for some fats.    One of my fat relatives wears her underwear high, practically up under her armpits (which is also where her belts go).  She told me once that if she wears it down around her hips, the elastic will chafe under her bottom fupa roll.  You know those rashes that they get from being sweaty and covered in bacteria because all their sweat and dead skin collect under there and the rolls rub it into some sort of yeasty rash?  Now add elastic bands biting into that.  So that's why she pulls panties up over her fupa - she'd be bleeding within hours if she let her panties fester under her rolls with all the sweat and gut-jam.