Sizes generally have more room for more bodies but tend to run on the the traditional bigger side.
XS fits like a small (women's size 4-6)
Small fits like a medium (women's 6-8)
M/L fits like a large (women's 10-12)
Curvy fits women's size 16-18
Curvy + fits women's size 18+
Model with baby is in size M/L, other models are in size XS