For reference, I'm slim up top and thick down below. I normally wear a size 8 top and a size 10 bottom. I bought a size M in this jacket and it fits perfectly. I can wear both a thick top and a thinner top and be able to zip up the jacket comfortably. Get it! (I should also mention this jacket is a great dupe for the one from Zara that sheds badly. I bought and returned it because of the shedding. I then got this one and I'm in LOVE.)
Beautiful jacket! Even nicer than pictures. Very warm, is a little big - but XS size is perfect to wear over a sweater. Absolutely love it!
Fits super well, and can layer inside. Love the design! Very comfortable.