Subconcious assumptions that this is perfectly natural and fine would be, in the eyes of feminists, a good deal worse than an overt attempt to offend. They're getting worked up because something that is blindingly obvious to them is completely oblique to everyone else, which in itself is a demonstration (In their eyes) that their views are correct.
Right. Light-hearted. That's totally how society treats overweight women.and theres no reason why a joke has to be shameful or penalising towards another person, jokes can just be light hearted