I've long been frustrated at the lack of bars in my preferred size, 40 cm o-o or 38 cm c-c. My shoulders are relatively narrow, and when I go to wider bars, I feel like I'm in a less stable position, with arms spread out. If I'm holding the plank position, I want my hands directly under my shoulders, and on the bike it's the same. This is the position of greatest mechanical strength.
On top of this, narrower bars are more aerodynamic and have less of a cross-section for contact with adjacent riders. It's win-win.
Yet for some reason wide bars have been popular. People feel they can get more leverage on wide bars. But power on the bike comes from the legs, not the arms (tests have confirmed this). To maximize this, you want to have as smooth and fluid a pedal stroke as possible. That implies staying balanced on the bike, not wildly thrashing about. So I think any benefits of wide bars except in violent sprints are over-rated.
But what about violent sprints? Andre Greipel is perhaps the most powerful sprinter in European cycling and he won the prelude race to the Tour Down Under in Australia yesterday. A key leadout man is Adam Hanson. Here's a photo of Adam at the race:
Adam is running 38 mm bars this year, the same size I prefer. And Adam's significantly larger. I copied the bars in front of his shoulders to show the bars are clearly narrower: