6', 175# here.  I can squeeze into a Medium, but I hate the feeling of tightness with every arm movement.  I'm very wide (I wear 36" waist pants, and have since HS when I weighed 140# and the muscle mass of a pencil.)  I wear larges so that the shoulders fit, and I can keep my shirt tucked in if I lift my arms. But, I HAVE seen the sizes expand over the years.  My XL sweatshirt from HS is smaller than the Ls I buy today, and that was 30 years ago.