If I view videos on Youtube, I see the fullscreen button, as well as being able to double-click the video.
If a youtube video is embedded on a non-youtube site however, I'm missing both the button and the double-click short cut. Double click simply pause/unpauses.
This is using latest stable Chrome on Windows when logged in. If I run an incognito tab so I'm logged out, the fullscreen button appears again.
Logged in using Chrome on OSX does acts as expected. I'm using the same extensions on both OSX/Windows. Disabling all extensions has no effect.
UPDATE: Some examples
UPDATE: Here's a screenshot