The aspect ratio can be from different things, and even includes the resolution that your set top box is set to. Try changing the resolution as well, and see if that helps. The links below will provide more information on this. Hope this helps!
- Slingbox.com - Why doesn't the video fill the whole screen?
- Slingbox.com - Video resolutions supported by Slingbox M1/M2, Slingbox 350, & Slingbox 500
Sling Media Moderation Team