There's probably a graph and complex mathematical formula to be worked on, plotting initial buttock roundness/angle against hip protrusion and the like. My personal hypothesis is that there's some interplay of dimensions that help trews 'stick' like a hula hoop on a jutted hip. Or something. Never been a huge problem for me I'll admit, but as I get skinnier and older I can see how it may be a concern. Hell, after years developing stupid shaped legs from sports that involved too much squatting, I'm still shocked to get my arse into a pair of 501s rather than baggy things. And, thank gawd, at least I'm not one of those high creviced freaks.