Beth, I would have said exactly what you say here:
Some stretch, others don't, even in the same general fabrication. So weird.
It must depend on the type of poly used, and a whole bunch of other factors. I can't figure it out. All I know is some jeans stretch and others don't.
Oh-- and even the same jean may "bag out" on one body and not on another. Which may have to do with original size bought and body type modifiers. I honestly don't understand it.
I have a pair of white Kut Dianas that definitely bag out after a couple of wears on me. Luckily in a good way -- I don't think they look bad when they get a bit of ease to them.
But others have the same Kuts and say they never bag out!
I have a pair of stretchy blend Mias that don't bag out at all -- they're in a size below my regular Kut size. They fit like super skinnies.
Then I have a different pair of plaid Kuts, bought a size up for slouch -- and they bag out like you wouldn't believe!!