I have 38s on my road bike and cyclocross bike - I have pretty narrow shoulders so this works quite well for me.

I also run a very narrow mountain bike bar (don't know the exact measurement) but since mountain bike bars do not have a lot of places to put my hands and sometimes I like to be wider for climbing, I have bar ends on the end of my bars for another place and position to put my hands.

When I started riding mountain bikes, it was the norm to have bar ends. Now people make the choice. I really have a hard time riding without them so I love them.

I tried riding riser bars on my mountain bike and that was a disaster. First thing I did was crash - and then I was determined to ride them to see if it was because I was not used to them so I rode them for 2 weeks. It did not get better. That is, my bike handling was horrible so I went back to narrow straight bars with bar ends and love them.