When Google take over YouTube it's a daily news of new features for their customers (for us). A new feature added recently allows us to play HD (High Definition) video's when we switch to full screen mode. This way we can enjoy the real fun in our video's.
Goto "My Accounts" and click on "Settings" then choose "Playback Setup" from the left panel.
Now Check "
Don't forget to click on "Save Changes" button to enjoy HD video's in full screen mode by default.