YouTube's Google Video-ization continues by using more space for the player and less space for the sidebar. YouTube changed the aspect ratio from 4:3 to 16:9, generally used for high-definition TV. Since the videos aren't yet converted to a widescreen format, you'll see an annoying pillar box effect.
"The pillar box effect occurs in widescreen video displays when black bars (mattes or masking) are placed on the sides of the image. It becomes necessary when film or video that was not originally designed for widescreen is shown on a widescreen display, or a narrower widescreen image is displayed within a wider aspect ratio."
{ Thanks, Mark. }