Nikon D90 and iMovie Exporting

Every time I attempt to upload a video to the internet (vimeo, youtube etc.) it works perfectly fine but the quality of the video is much less then when I view it on the iMovie preview screen. I use my Nikon d90 camera and the iso is 320. When I export the videos i click publish to media browser in large.Am I missing something? I know there's an HD setting but I'm always unable to click on it.

When you export, keep in mind that the Nikon D90 video is 1, 280 × 720/24 fps.