Purchased two XXL polos and ordered a size a little too small. Ya, I know, "not as lean, but still as mean!" Ok, so I called for an exchange (XXXL) and they were very helpful. The order was adjusted, returned and shipped out with the larger size. They fit great!
Bottom line...if it has been over 30 years since you did a 3 mile run and it looks like you are trying to smuggle a basketball out of the gym, I'd go a size larger.... Great shirt quality and customer service! Thanks Leatherneck4Life!