I'm 5'7, 160 lbs, 38 chest, 31 waist, 15.5 32/33 shirt, non-muscular upper body. And I wear almost everything in small. Haha, for a second there, I thought those were my stats again. Must be my broad shoulders from all my lifting that is keeping me from fitting into the S. Just out of curiosity since you have the same chest measurement as me, are the secret wash shirts a little tight in the chest area when you button all the buttons except for the collar? I just checked the J Crew website, and they actually recommends size M for those with 38-40 chest. That might explain why the M has some breathing room for me while the S is kind of tight (to the extent that I can actually kind of feel the shirt pressing against me). Nah, they are different. Your stats(10'' drop) suggest that you have a much more athletic build than I do. Strange enough, I don't find the chest tight. But as you mentioned, I do find that some shirts tend to have special wrinkles around the 2nd button. I own a medium j.crew and actually swim in it. Not sure if it's because it runs big than normal mediums (it's a flannel). Overall, I like how smalls fit on me.