Cannot download the original quality of some 4K video on youtube, the downloaded version is highly compressed

Cannot download the original quality of some 4K video on youtube, the downloaded version is highly compressed

For example, the link below, I think it is due to HTML5, becasue when I use flash to play this video on youtube, it is still highly compressed, unless I turn to a HTML5 version.

https://www.youtube.com/watch?v=Qs3OL3GzZms