If you are worried about your data plan, it is better to turn off loading HD videos on YouTube for Android. To disable HD on YouTube, first open the app and then do as follows:
1. Press Menu Button and go to Settings
2. Under General settings, make sure High Quality on Mobile is unchecked.
*** You can also find the video, rotate the screen to enable YouTube full screen mode, and then tap the HD Button left to the progress bar. It toggles HD loading On/Off