I don't know how old you two ladies are, but I'm sure age, general frame size, and hormones, and bone density have something to do with actual body weight.
My aunt, who is my height (5'4"), and 70-years-old, and in pretty decent shape, is a solid size 14. She weighs 25 lbs less than I do. (She weighs 150, while I weigh 175.) Also, since she's 70, she doesn't wear jeans the way I wear jeans. She prefers them more loose-fitting, whereas I prefer them more snug. Anyway, my point is, we own the same pair of jeans. I actually gave her the 14s when I could no longer fit in them and then bought myself some 12s. And now I sport some 10s in the same pair of jeans. Her body is quite different from mine though. She has rather wide hips, but practically no ass. According to her, she's never had a butt. I, on the other hand, have wide hips and (according to my boyfriend) am "rumpy."
-Tamara