Amanda - so when you ride your 38's, your hands kind of flare out to the sides (if your arms are straight) when you are on the hoods? That does sound a little like a hood positioning problem.

But, I can see how if your chest makes your arms spread wider than your shoulder width, then measuring your shoulders wouldn't be the right width for you. Do your elbows point out to the side when you ride, or are they pointing down? That might might point to what it is that's causing your arms to be wider. If the elbows point down, I would think your shoulders are wider, but if they point out, it would be the chest causing it. This is totally non-scientific. I'm sitting here at my desk trying to figure it out using my own arms, so it's only a guess.

I ride 38cm bars. My first bike had 44cm ones and while an open chest is good for breathing, it's not so good for aerodynamics. I felt like a sail with my arms spread so wide! I was also having upper back pain. As soon as I switched to the 38's, it was instantly better. I put the same width bars on my commuter, and it definitely works for me (but I am not chesty at all).