To sum it up. Measure yourself. The jacket will come true to the size chart !
I watched a lot of you tube before picking my gear and on this jacket there was a lot of mention about size which had me confused. I usually wear a large shirt and/or jacket. And I was led to understand these run larger for layering. The question that kept running in my mind was "how large is large with extra for layering?" SO, I measured myself and went with a medium. Glad I did too because my layering will consist of a thermal undershirt, a light weight shirt and this jacket under a shell. I won't say it fits snug because that may be misleading. It fits perfect for what I want. The arms are long enough (and I have long arms) and I have plenty of room for movement in the shoulders. I love that it is not baggy anywhere. The wrist bands are tight but this too will be handy if I fall forward into the snow or if it's raining. The waist band is also fairly snug so I won't be getting any draft or folding under my pack belt. I haven't had it out in the cold yet as I live in Southern California but from the other reviews I am confident I can start a thru hike with this jacket and not worry about anything.