You have probably noticed that there is no standard for what sizes mean. I live in a world in which most size small items are a bit too small, and most mediums are a bit too big, but some size small items and some medium items fit me just right. Since I expected a down vest design to be roomier than a shirt, I went for the small. Well, it turns out that I find it a little bit tight when zipped up. So, if you are between sizes, I suggest you go for the larger of the two. That aside, it feels good, warms without over-warming, and looks good.