I think it is something to do with your graphics card... I am sure there is an option (or atleast there is on mine) to scale the image to screen size.
What graphics card do you have?
Sam
Exactly this there should be an option in there to scale full screen.
1. Go into your Resolution options. ( You'll want to know what res is native to your game )
2. When you have changed the res to what you need, Click advanced settings.
3. The top tab should say something like "Intel(R) Graphics and Media control panel", Click it.
4. Go to the scaling drop tab and select "Scale Full Screen" and Apply it.
5. Go back to your Resolution options and set the screen back to your preferred Resolution Settings ( mine Is 1920x1080 )
6. Test your game to see if it worked
For intel.