Is your monitor's aspect ratio the same as your output video's aspect ratio? Because differing aspect ratios will cause this.
For example:
If your monitor is 1600x1200 (4:3) but your output resolution is 1920x1080 (16:9), you'll have black bars at the left and right sides.
If your monitor is 1280x720 (16:9) but your output resolution is 960x720 (4:3), you'll have black bars at the top and bottom.