These jeggings are super cute! I ordered the purple color and am considering buying more because they are so comfortable, and flattering. I have a big booty and have trouble finding jeans that work with it, especially skinny jeans but these fit much more like leggings that jeans and the fabric is super soft. I  am 5' 4" and weigh around 145-150 and wear (usually) a size 8 and ordered a size large. when i took it out of the package they seemed really small, but they stretch a lot and fit great. If i had a smaller waist i could get away with medium but i wanted a looser fit for a smooth look under sweaters.
Overall i would totally recommend these jeans and if you can't decide between two sizes i would go up just because the materiel is so stretchy it would look better to go bigger than smaller. Hope this helps!