Hover over YouTube video thumbnails to retrieve their stored zoomed and expanded full quality size of 480x360px
Note
After YouTube's new material site design update, this userscript no longer works. However, I found an alternative script that does a perfect job and would recommend it.
Mouseover Popup Image Viewer by tophf and wOxxOm
https://greasyfork.org/en/scripts/394820-mouseover-popup-image-viewer
Ironically, the 'Mouseover' script doesn't work on YouTube's older site, which is why I made Youtube Thumbnail Enlarger, but it now does work with the new site design. I will leave Thumbnail Enlarger up for those who use YouTube's older style:
Info
YouTube stores video previews at a large size then reduces them to fit normal page listings, this script will retrieve and display the original large-sized thumbnail.
