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.