Only a seeing a quarter of video screen in Youtube

When I'm using YouTube, and try to go to full screen mode, I only see a quarter of the full screen. That quarter of the screen shows all of the video, it just doesn't fill up the whole screen of my computer. I only just noticed this today, and I updated my computer yesterday (Safari had been updated three days ago as well), but I don't know if that was the problem. I've tried updating Flash Player, restarting the computer, I also found this, but none of it did anything to fix the video screen.

The video that I was watching earlier (when I discovered this issue) was live, and had a stream from another website and that one did work, so this issue is just on YouTube and only with the Safari browser.