blah789

Your caption says 1680x1050, which I assume is the native resolution of your monitor and also the resolution you're capturing at. Youtube in HD is 720p (1280x720).
What I think likely happened was it was trying to keep the aspect ratio (the ratio of the width to the height) and letterboxed it.

Did you use WME (aka Windows Media Encoder)? What settings (in particular resolution) did you pick for your output?

Your options
If you want it to fill out the whole area, it will be stretched sideways (in that case uncheck the box "keep aspect ratio" if there's one somewhere). The second option is to letterbox (add black edges to the top and bottom, or sides so the whole image fits in, but the aspect ratio is preserved), which is what you originally got. The third option is to crop, meaning you cut out some of the top+bottom or sides in order to make the aspect ratio match that of the target; in that option, you're removing some of the image.

Doing a little math:
your original is 1680x1050 so the aspect ratio is 1.6
a normal 720p video is 1280x720 so the aspect ratio is 1.777...
1. If you want to fit your video into 1280x720 it's going to get stretched horizontally
2. If you letterbox to keep the aspect ratio your output is going to be 1152x720, with black edges 64 pixels wide on each side
3. If you want to keep the aspect ratio but want to stretch it to the edges, it's going to be 1280x800, meaning 40 pixels are going to be cut out of both the top and the bottom (for final result of 1280x720)