h&m sizing is all over the place. I've been shopping in there today and bought a size 10 blouse and a S tshirt (I'm a 12), then a cropped jacket I couldn't get into anything smaller than a 16! A size 12 maxi dress and a M (they say 14-16) pair of shorts. You really need to try it all on.
Crazy sizes. I'm a 12-14 normally, more 12 than 14, 34E bust. Tops in S or M fit me, I tend to go M as I like the slouchy look. Trousers I'm a 14. Jackets I'm a 16! M is supposed to be equivalent 14-16 so assume S is 10-12.
H&M seem to have made their sizes larger. I usually take in a size larger than I would in any other shop, but everything I have tried on recently has been absolutely massive. I'm most definitely not a small sized person, but the mediums were all too big!!