Just got a medium sessions TNT - I measured myself up and I was at the upper end of the measurements for the "medium" sizing category, with some measurements (like my fat hips

) being at the absolute lower end of the "large" sizing category. The jacket fits slightly loose. Not tight, not fitting, not baggy. Just slightly loose. Length in the arms and at the waist is good. I'd expect the Large would've been way too big for me and that the Small would be tight.
I have to say the style is very oldschool though - almost too oldschool - looks like the kind of jacket the hobos in movies wear when they stand around bin fires.