I also have the prozero in a medium and it is a little more form fitting than the prozero. I went off of the sizing chart when I purchased the shirt. I certainly needed a small in the warm and probably a small in the zero. The zero is a little more forgiving in the sizing. I went by the chest measurements for sizing and it stated that a 34 inch chest was a medium, but I also have a very small torso (29 inches at the band) so I should have sized down for a snug fit.

I don't know if that helped you any at all.