Some streams are sized differently than the display on which they are shown. Just as televisions give you the option of using different aspect ratios to resize the image to your screen, AirServer also gives you this option by clicking the aspect ratio button during mirroring in fullscreen mode. Continue to click the aspect ratio button until you are satisfied with the image on your screen.
Please note that this is a feature only available on AirServer for Mac. It's only available during mirroring in fullscreen mode.