I have the field jacket LT and it is a bit slimmer than usual, but honestly it does feel like a "true" American large, just slim. I wouldn't call it an American medium.
It depends on the jacket, too. For example the Align shell is roomier than the field jacket.
Honestly, while the jackets are slim, they are cut so anatomically that you don't need as much room as you would with a normal garment.