That's what I've always thought was my issue. I have that issue with tights and leggings. If I buy according to the size charts, they are baggy in the butt and too long . . . and they walk down. If I go down a size, they aren't baggy, but they still walk down. I blame my flat butt for not giving them anything to grip.
I am with Cu5i. I am small, but with hips and a larger butt, and I have the same exact issue. I thought the same thing: it was my larger hip/butts.
I'm glad this seems to be a universal problem because it means there must be some fix to it.
So far the suggestions I've found are:
Legging brand loyalty (though no one says which brands)
Replacing the elastic with round elastic from the craft store.
Wearing underwear (or spanx for no lines) over them
staple them tight (at either side of the hip)
I haven't worn leggings or stockings in many years.. but I do recall that I always had issues with sagging/bagging/falling more in winter than in summer.. and often wondered if it had to do with the temperature... ???