Dopey, Manton--I don't think your own shoulders rapidly expand in radius toward your armpits. That would be really weird. So, if your jacket is doing that, it cannot have anything to do with naturally fitting your body. Just the opposite.
Here's a test Manton can do, since he has both NSM and Solito. Wear a Solito jacket, and feel the shoulder line just an inch or two short of the sleevehead. I bet you will find no empty space above your own shoulder (not counting the space created by extending the shoulder horizontally). Now, put on the NSM. Feel the same area. I bet you will find there is some emptiness above and perhaps in the front and back. If both jackets are unpadded, it is simply impossible for them to express such different shoulder shapes unless one does not keep as close to the body as the other.
EDIT: By "clavicle," I mean "acromion."