Sometimes, especially on laptop and flatscreen monitors that do not use the 4:3 ratio, the monitor itself will have an option to scale or stretch the display to fill it. Video drivers are also crucial in having this work properly, and even the game itself is responsible for it. Your options are to try and fiddle with the ratio options via the monitor, the video drivers, and/or whatever is available in the game itself. Sometimes even having video drivers that have a bug or are broken can cause the problem. I've often fixed problems with older games like Starcraft on laptop monitors being displayed in a small window at the center of the screen updating video drivers or just uninstalling and reinstalling them.
Any of those three elements - video drivers, monitor, and game - can have an effect on the display output of the game. You'll want to work with each to get the best behavior available.
