That is normal behavior. You can do one of two things.
Either play the game at your monitors native resolution, 1680x1050.
The other option is check in game options to display game in "full screen" or disable "window" mode. If it is not set to full screen, than it will only display in a window, for any resolution other than the resolution of your display. Again, this is normal behavior.
If you are playing at a lower resolution than your screen resolution for performance reasons, than it might be time to consider upgrading your video card. Scaling resolutions to full screen does not look pretty on LCD displays.