My wife and I each own a thermawrap jacket and I have a thermawrap vest as well. my jacket is from ~3 years ago and in size medium it feels to me more like a medium-small. I'm normally a "size medium" kind of guy, and this fits me, but very snuggly. Right now, with winter over and I'm overweight, it's a particularly snug fit. The size large fits fine, comfortably. Some size large things feel quite large to me, this feels just comfortable.
Bottom line is that I would be inclined to size up any montbell clothing purchase. We got the size large jacket for my wife, she normally wears things a bit smaller than I do but the size large is comfortable for her.
One nice thing, however, about having a size medium jacket and size large vest is that I can layer them for colder conditions.