To continue my string of posts, I'll now employ a crude graphic device in hopes of making things more clear.
Let's say this is the shirt you're staring with. (Pretend it's a button-up; this happened to be the first simple line drawing I came across.) The shirt is great in that it fits well in the collar and shoulders, but it's much too large in the arms and torso:
Here's how the tailor raises the armholes and trims the fit:
Leaving you with this:
It's an easy alteration. The important thing, as I mentioned earlier, is that the shirt fits well at the collar and shoulders to start with. I've had little trouble with this; shoulder size increases with overall size, yes, but not as dramatically as you might imagine. The shoulder seam might not sit exactly perfectly at the corner of the shoulder, but it won't be that far off, usually. Just something to look for when buying a shirt.