I'm 155 lbs, 5'7" and the medium fits very well. I wouldn't want to go any smaller, as the chest is almost too tight. It is a bit disproportionately roomy in the lower abdomen, but the cinch cord works well and more than makes up for it. The hood cinching system could be improved, as well as lighter cordage used, but it's much better than not having anything there.
Weight came in at 8.35 oz. @ size M in Rust (great color!)
(and .2 oz. for the stuff sack)
It says right on the tag:
Europe M / Japan L, so it is western sizing, but on the "athletic" side still - I'd order up a size if it is to be used as a belay parka.