There was a move for several years in racing to wider and wider bars, but riders then figured out they were going the wrong direction.

First, leverage: in fact, your leverage is applied very close to the center axis of the frame (i.e., pedals and saddle), so if you have bars that you're holding way out to the side because of the width, you're just having to swing back and forth on the bike to make use of the leverage. It's like trying to pick up a heavy weight at arm's length -- instead, do it closer to your body.

Second, aerodynamics and lung capacity: With narrower bars, your aero position improves (that's the whole point, at the extreme, of aero bars). And if you use a slightly longer stem with the narrower bars, you stretch your torso slightly which in turn causes your rib cage to rise farther above your diaphragm. That's the best way to get more oxygen into your lungs.

Third, shoulder/back support: Your shoulders and arms are supporting your upper body on your bars. If your contact points with the bars are wide apart, then your upper body will basically want to fold down between your shoulders. This leads to wasted energy, poor position, and potential back, shoulder, and neck problems. Again, think about doing pushups with your hands spread far apart, versus close together. You'll prefer the position with your hands closer together.

I'm not suggesting going to 36 cm bars for everyone, but sports physiologists took measurements of all the athletes on the US Olympic team and found that it was a rare individual who's shoulder joints were actually wider than 42 cm. Even on a hulking discus thrower or wrestler, any extra width was due to muscle or bone on the outside of the joint. The vast majority of individuals were 39-40 cm wide at the shoulder joint. These kinds of measurements were duplicated when measuring thousands of college students in a similar (non-athletic) study a couple years later. So whatever your impressions of your own shoulder dimensions, be aware that very very few people really are built wider there -- it would create problems with all the rest of your musculature, skeletal system, and internal organs. I've tried to convince myself a couple times that I needed 42 or 44 cm wide bars, but my doc finally just showed me an x-ray of my upper chest, and guess what ... 39 cm. He's skinny as a rail, but he was 39 cm too. His partner, who weighs over 300 pounds and was a tackle in college football, was also 39 cm. Interesting discussion.