I have had many younger customers recently coming in to buy shirts that are fitted much closer to the body. I have found that in many cases they are wearing under shirts that are too big. This causes fit problems under the arms and behind the collar. I do not wear an undershirt under my dress shirts. The wearing or not wearing of undershirts is another topic. The point is that if you want to wear a fitted shirt, make sure you are buying undershirts that are also fitted close to the body. Maybe even a size smaller then you think you need. This will eliminate seeing the lines of extra fabric under your fitted shirt. Also make sure that the neck line of the undershirt is not too high.