Handlebar width is about leverage and control of your bike. I have not found it to correlate with shoulder width at all. If they feel wide, then get narrower bars.

If you want to generalize, sprinters prefer narrower bars, climbers prefer wider: sprinters don't want their bikes going side to side in a sprint, so go narrow; climbers want leverage, so go wide. Also, narrower bars are not only more aerodynamic, but also allow you to ride closer to other riders. Narrower bars also make it easier to protect your bars with your elbows in a race, if you are into that sort of thing.