I think subject A looks bigger because the second picture is more tightly focused on her, so she's taking up more of the 'room' in the picture. I do think her butt looks better, though. Less lopsided.
Jeans are hard! If it fits my stomach, it is too big in the legs. Skinny jeans don't seem to work, but I'll give them another try, maybe in a size higher?
Lately, uhm, I just buy them at the thrift store. Hard to justify spending $80 on jeans you hope to wear for 3-4 months. Instead, I venture $6 if it seems to be in my size. Though uh, lately I've found a number of thrift-store jeans with the size labels torn out! HELLO, people can still tell you're overweight even if you tear out the damn label. I have a hard time eyeballing since I overestimate the size I need right now, so I never buy those.
Thanks for the link, though, I never spend that much time looking at my rear in jeans and now I am going to check that view, too.