As for me, when I use wifi the videos on youtube go into what looks like 1080p, and a "HD" indication is shown on the bottom left, but when on mobile date "4G" it goes to 480p and below and sometimes 720p depending in what area your in for 4g strength, and an "HQ" indicator is shown on the bottom left instead of "HD". I'm guessing this probably happens because of my mobile data limit on T-Mobile, resulting in the youtube videos going down in quality depending on where the internet is coming from wifi or 4g.
This is quite annoying to me, does anyone know an app that allows you to chose what resolution you want to wach a youtube video on for example 720p or 1080p on mobile data?