Billie Eilish did this and posted it on her Instagram. I really don't know what it's for, but she included some emojis that probably explain it, and I'm not deciphering that like I'm an archeologist, that's not my job. It was said she has a "jaw-dropping figure" and yes she looks pretty good these days, but that's what happens when women grow up. They look like women and men start to find them attractive. That's how biology works.
Her first song was released when she was a child - she was 14 years (ish) old in 2015. So of course at 21 she looks "better" And she's still basically a child today. She's still only 21 years old even now. I don't even remember being 21, can you imagine talking to her about anything? She's been famous since she was a freshman in high school and probably only learns things from memes on the internet. Really I don't even want to think about it.
