The videos don't immediately show up as High-definition when uploaded to YouTube. You may be able to see 240, 360 or 480 versions, buy you have to wait a while before YouTube processes the Hi-def versions.
Even when they have been processed they don't show automatically as Hi-Def, as many YouTube viewers are on slow modems (or dial-up), so the videos generally start at 480 (unless smaller-sized).
You have to select the quality, by clicking on the number which shows up immediately below the bottom-right of the video, and then you can change it to 720 or 1080, if the option is available, and finally click to full-screen
Trevor . . My YouTube site . . (Bristol, UK) . . My JVC GZ-HM1 Camcorder