Unfortunately the more famous these huge women get, the more pressure they get from the world to lose weight. Certain levels of fame mean they have dieticians, personal trainers, or even doctors and surgeons who want to be the one that helped them lose the weight for advertising purposes.
Also: it's rare for a woman to prefer being fat to being skinny; most of these SSBBW models are just making the best of a bad situation. From there it's rarer for a woman to prefer being incredibly, comically huge. Rarer still for a woman to feel compelled to get bigger and bigger on purpose, especially the ones who start off skinny and conventionally attractive, who decide to ruin their own bodies. Rarer than that for a woman to be sexually turned on by her own body getting so fat to the point of being handicapped, health problems, rejection from society/friends/family, etc.
Most of these SSBBWs would, if they could press a button to become skinny, would absolutely press that button.