winter jacket sizing?
 

I am thinking of getting a Pearl Izumi Zepphr jacket. I don't really know how these jackets or jackets in general fit. I normally wear a medium (38-40) jersey. If i were to order this jacket in the same size (medium 38-40) would it still fit? Do they adjust their sizes to account for wearing clothes underneath? Does anyone have any experience with this?

I don't want something way to big. But it's not that big of a deal if i have to exchange it for another size.