Why is it that the video quality on imovie 09 or imovie hd looks good but horrible on youtube? Help?

i record with my isight on my mac within imovie 09 or imovie hd. whenever i play it back on either one, the quality is GREAT. on youtube, not so much. how can i get really good quality without using "hd" or widescreen. i want HQ not HD. thanks