I wear a 39/40 jacket. The sweater that I am wearing is a 40. As you can see, there is extra material in my upper arms and chest. I wish I could have had the opportunity to try on a 38 to compare. Of course, the danger is not having flexibility to move the arms. The sleeves are very close fitting. You would not want to wear a collared shirt under this sweater. (Besides, who'd see it?)
This is a heavy sweater. Underneath, I wear a Patagonia base layer. And that is plenty warm. Yesterday, the wind chills were in the lower teens, and this sweater, worn under a light shell, offered perfect protection and warmth.