Dennis-

If I remember (can't find the source, so I'm not sure), she had to be that narrow because of the high speeds she travelled at. The smaller diameter fuseleage gives her less resistance and thus allows her to fly faster. I guess the cramped interior was a result of that.

If you want to learn more about Concorde, check out this site: http://www.concordesst.com/. It's a great source for information about everything dealing with this amazing plane.