They don't actually, except for runway models, maybe (but they're practically walking clothhangers - really creepy!), fashion models are more like 80-60-90 (or even 90-60-90) and 175-180cm tall. So they're not extraordinarely thin, but their height makes them look extra slim in photos.
Size 00 is mostly worn by teenage girls who are still build like little boys and girls who really want to believe they fit in the smallest size available.
And/or they're shorter at the time. I'm 5 (or 27, depending on your sizing scheme), but I'm short, so I don't look model-skinny. But if I was closer to 6 feet and still that size, I'd look ridiculously thin.
The other downside: pants are always too long on me and I always have to get alterations. I've tried stores with petite sizes, but I never really like their style.