>>125316
Japanon, I am a /fringefitlit/, and I can tell you that biology is alterable.
Expand your chest cavity from the inside out with your breath; when you can do that, feeling your miserly little tendons straining as they stretch to accommodate the large lung capacity and neuromuscular connection to your rib and general upper body muscles, you will notice that your collar will stretch as well. The real problem is the arms. You have to longitudinally stretch your arms out. Do dead hangs, pull from a door frame, use a broom or stick to do normal shoulder stretches, and learn to stretch in a meditative fashion in order to prevent tension that will only slow your progress.
I accidentally started doing this by cultivating the "Metal" element of TCM. The body constitution for Metal types consists of the largest shoulder width amongst all body types.
Good luck, and try the Metal thing as well.