It's mostly a meme, SLP denim is actually specifically designed with the goal of making more normal body types seem super slim.

Don't get me wrong, the campaigns and shows are filled with actuality emaciated models, but that's just part of the marketing (and fetishism, par Hedi.)

There's a reason so many celebrities wear SLP - as long as you have a fairly average, well kept body, they'll make you look good. Models just make them look particularly good.