It's a point I've made elsewhere, but the relevant width to think about in measuring your shoulders is from center of ball joint to center of ball joint. After all, bulking up your deltoids shouldn't change your position, right? Now here's the interesting thing. There are a few measurements in the human body that simply don't vary that much regardless of height or build or ethnic origin. That ball joint to ball joint distance is one of those and it applies to both shoulders and hips. Your shoulders may extend much farther than the next guy, and your pelvis might do the same, but the ball joints themselves are pretty constant. If you think about it, there's a lot of geometry involved in those joints and evolution may have decided to keep the distance constant so it could keep the angles and lengths and other dimensions of your femur's or humerus's trochanter the same. Otherwise you'd have a lot of people with malformed joints. So keep the dimensions all the same and it's a simpler matter for evolution.
Where this takes you is that most people have pretty close to the same ball joint to ball joint distances, and they tend to be in the range of 37-39 cm. The Italians messed everyone up with an amateurish fitting guide in their CONI manual back in the 1950s, and the whole world adopted it. The Japanese keirin manual actually copied the illustrations and translated the text so literally that the Japanese even has a bit of Italian phraseology to it. Anyway, it means that wide bars were basically an idea promoted by some Italians who didn't really get it right.
So this begs the question of what is really right. Nothing says your bars actually have to be the width of your ball joints. Some people have range impingement of various types, especially in a cycling position, so that sets limits on your position. Others have body mass issues, or joint deterioration, or whatever. Reasons for narrower bars are more common and reasoned than for wider bars, by and large: You can work through smaller holes in a pack, you provide a smaller frontal section to the wind, narrower bars are significantly stiffer and also lighter, you are pulling more in line with the force you are applying through your pedals, and so on and so on. I happen to like narrow bars, mostly because I use 34 cm bars on the track and wider bars feel ungainly when I get on a road bike. But it's horses for courses, and in the end, this is like all aspects of fitting -- you have to know yourself. I like to promote the idea of looking at narrow bars, mostly because the Italians have had it all their way for 60 years, and that's reason enough. Now if it had been the French ...