I don't think it's a browser issue. When I run in full screen mode I have large areas of dead-space (grey background) on both the left and the right of my screen due to my landscape style monitor as well. The problem likely comes from the use of static width objects that are tuned to work for the lowest common denominator amongst all of the user's screen resolutions. Building the layout so that it adjusts to your resolution is probably somewhat more challenging.
Looking through some of the other pages I have open, I see exactly the same issue: Facebook, Howstuffworks and another informational forum hosted by Amazon all make poor use of the available margins. In fact, the only exception is Google news.
Most computers have landscape-style screens, wider than they are tall. Chess.com's layout is portrait-style and doesn't fit properly. It annoys me that half of the stuff I want to see is out of sight, but I have two huge empty panels on either side of the page. Competitor sites appear to have arranged their layout more appropriately.