They may not be the same shorts.
Mine have a quite small gusset, which really only serves to avoid the 4-seams-meeting.
I do have the creeping up the crack and crawling up the legs problem when I wear a size too small. (I have 3 sizes of UnderArmour shorts for the 3 stages of healthy-chubby-fat I cycle through.)
For what it's worth, my butt is quite a bit larger than just 9 inches difference from my waist. Always has been, always will be... "the eternal hourglass" no matter if I weigh 145 or 205.
Currently I'm 205 lbs, and wearing my XL UnderArmour. When I get down to 165 I switch to the L, and so on.

