I always wear a small/xs and have a white shirt from Reiss in a small (the Buster style). It is very slim through the body, however I think this is very flattering as the shirt is meant to be tucked in, and it effectively leap-frogs the issue of having tucked in shirts which bulge over excessively when you lean down.
Obviously I cannot comment for how a large would fit, however I would imagine this is a trait common with their slim-fit shirts.
How do you find the length of the shirt you have? My one is long because it is intended to be tucked in, however I have always been put-off buying shirts from Reiss, fearing that their cut leans towards middle-aged, frumpy men. They have certainly helped to drop this aesthetic in recent years, so maybe I will have to invest in some more Reiss shirts when the sales start.