This sometimes happens. All videos have to be compressed to allow us to stream them quickly, otherwise the load time for people who want to watch your video would be so long that they'd nod off to sleep. So sometimes they don't look as good as the source. Don't be concerned if you uploaded your video for a competition and you're not happy with the quality, because (unless otherwise stated) we judge from the original, source material, rather than the version that's on the website.