No one has ever described my butt as muscular... (although it is a lot smaller than before I started cycling), but I do think it has something to do with having a "cycling" body that is slim and fit, rather than someone who is just skinny.
I never thought the problem could be that the shorts are too big, though. I have a pair of boy short swimming bottoms that are size small (my usual size for a swimsuit) from Athleta and they do the same thing. I wear them canoeing on the neighborhood pond, but that's about it, because I am so uncomfortable in them.
Knott, I could see how these boy shorts might rip easily. They are very thin, but warm.