Imovie pillar boxing

[Zepck]

Hello, i've been using iMovie 11 lately to edit screen recordings using the application screenium. I record with no compression and my screen size is 1280x800 on a 13 inch Macbook Pro. When I create a new project I make sure that widescreen 16:9 is selected. I am unsure where the problem is, but after I export in 720p the final video has large pillar boxes. Black bars on both the left and right sides of the video. Also the pillar boxes are visible on the screen to the upper right when editing. This is really annoying and I would really appreciate it if someone posts a solution. Thanks in advance!