Look at this screenshot and see if you notice the changes (it's in full screen):
See the little black spot on the timeline? that tells you the timecode where your mouse is pointing allowing for more accurate time selections (if someone tells you watch this at 43 seconds or something like that).
Also, this might less noticeable, the full screen bar at the bottom is about 50% of what it use to be.
Also, when the movie ends and the video zooms out so you can see the link and embed code and related videos, it zooms out less when in full screen.