First off, I have numerous Castelli products in my closet. Some fit great, some fit okay and others I send back. This jacket fit better than the Mortirolo jacket, medium and large, that I ordered and sent back. I also ordered the Raddoppia jacket with the Mitico jacket to make a fit comparison. In my opinion the Raddoppia jacket fit better under the armpit and would work for the range of temps that I will wear it in.
The Mitico is comfortable, stylish and well constructed, but it is snug under the armpits....not uncomfortable like the Mortirolo was. If the Raddoppia jacket was unavailable I would have kept the Mitico. Like ski clothing, you have to try it on and determine whether it will be comfortable for a couple of hours. I sized up to a large based on previous experience with Castelli garments.