Skirts in
sl are made based on a model with the sliders set to 0 in the ass and leg areas. They don't fit me, and even if they are modifiable it's nearly impossible to get the same look as what they were advertised. So, I simply don't buy prim skirts.
Also they look terrible when you walk since they can't follow your leg movements, but that's just my opinion.
Runway models look like they do because they are basically hangars. They are there to show off the clothing. It's the same in
sl, except in rl when we buy an outfit we can get the size that will fit us specifically. That doesn't work in
sl without a lot of effort, and a lot of times not even then. Still, this isn't a thread about that, so sorry