Why do I envy naturally slim women?

I've always been a big girl my whole life and unfit. I lost weight but gained the wait back. Some of my relatives says I don't look good slim. I'm big built. When I see a healthy, fit slim woman I get SUPER JEALOUS but these days most women wanna gain weight and get thick. I've been big and I'm going to get slender and I don't care what men think!