Ah, I see you're a fan of the blanket statement. You might want to go round Insta and check out the body positive crowd. I got called gross for discussing my efforts at health so I can guarantee there are plenty of them not calling the clothes "slimming" at all.
Could be countersignalling. If you wear baggy clothes, people won't know if you actually are big or if you are slim under those clothes. But if you wear slimming clothes and still look fat then you must really be fat.