I have a similar problem. I have a healthy, moderately athletic body shape, and most RTW shirts give me muffin tops and "flying squirrel" armpits.
My experiences are:
- C&A "slim fit" dress shirts fit me okay with neither of the above, but I need to move the collar button by a notch to have a comfortable "insert two fingers" fit on my neck. Since I'm on the small side, I need to go with XS, which is pretty rare.
- The absolute same situation with Uniqlo slim fit shirts. They are pretty cheap, but quite forgettable both in fabric and cut, the C&A ones are superior. (Though Uniqlo readily carries XS, while C&A may have them once in a leap year if lucky.)
- I also tried Topman slim shirts, which ARE rather slim, but far too slim in chest too, so that's a no for me.
Everything else I've tried was too big and bulky, even in their minimum size, both in Europe and New York. I was planning to check out Suitsupply's extra slims but I haven't gotten around to it yet. I guess the only REAL solution is MTM.