Because they're so proud of being fat that they have to usurp the meaning of word after word that even vaguely means or implies sexiness and non-fatness.   See also cute, fierce, goddess, un-thin (shit you not), and petite (shit you not again).  Hell they're even harassing actual anorexics on anorexia forums insisting they are curvy fucking anorexics and demanding that the below-healthy BMI per medical definition anorexics "prove" they are who they say they are because they get triggered by their photos (just learned that tid bit last night), so much fucking hate.