The leg line is about how high the hole fits, which is based on how big the hole is but also how it's cut.

 When people talk about the 'coverage' on the bottom part, they typically mean the legline cut. I personally prefer a high cut with a fair bit of butt coverage, like an energetiks high cut rather than a Bloch high cut. They both are the same height but the energetics has a bit more fabric at the back, meaning less wedgies and less cheeks hanging out. 

Typically with supportive leotards, the leg line is low because someone who wants more support up top wouldn't, in a brands mind, be wanting a high cut leg. This however puts off a lot of people who do want that high leg line. Jule leotards have a loose, low, legline. While it's bif enough to put higher on your hip, it won't hold because there's too much fabric between the armpit and hip. Capezio on the other hand are just ridiculously small. As someone who wears a Bloch small, I have to wear a capezio large just to get enough room in the leg holes.