im assuming you have a widescreen monitor, i have this feature on mine, il take a few screenshots with my phone to show what it does. this picture is the scaling options for flat panel monitors:
now, with this enabled, at your monitors default resolution, will appear like this:
now with it set at a resolution that isnt your monitors default will appear like this:
to stop this from happening, simply find out what your default resolution is for your monitor, and set the application resolution to this.
if you cannot find anywhere to set the default resolution, you can have full screen by reading through the manual for the monitor, there will be an option to remove the aspect ratio scaling on the monitor somewhere if you cant set it on windows.