How come people that were once thin now look chunky or thicker?

Like me for example I was tall and thin until puberty and I gained so much weight. Now I’m 249lbs and 5’11” 3/4.

My sister was skinny and she looks amazing now. She is beautiful no matter what size. She said, “Can’t believe how skinny I was back then and thought I was fat. Glad I don’t look that skinny anymore.” She was looking at a photo of herself when she was 13-14. She is 35 almost 36 now married with three kids.
My sister is slim but curvier.
I want to be slim again but keep a shape.