Even a size 6 with a nice well defined behind would barely look half decent wearing this style.
I've got a fantastic back side and would never put it on display like that. I would not have done it when I was 18 or in my 20s either.
It looks very indecent. Why would anyone want to showcase and display their body like that is beyond me. Like it is for the 'consumption' of the general public.
And the aesthetic is so bleurgh. They'll go out of fashion at some point I'm sure.
I've only ever seen them in real life once, in a random Tesco I stopped at to grab a small shop. 2 lovely looking very young women but the whole ensemble looked cheap, uncomfortable and weird and didn't do either of them any favours. At least the leggings were black, not nude or some bright colour to draw even more attention.
I'm not an advocate of everyone can wear what they want. No, people should dress for comfort, style, decency and appropriate for the occasion. And, by all means, to enhance their look if they so wish. Can't buy good taste unfortunately.