YouTube is among the most commonly watched website where people log in to see videos. The videos are available in multiple qualities and user can select an appropriate format for playback as per their internet connection. 

For example, if you use high-speed internet you can play videos in high definition formats without pixel break or choppy playback.

YouTube now provides its user with an option to select the default format of playback on their PC so that they do not have to change formats before playing videos. If you have slow internet speed then you can watch videos in lower quality by selecting the options manually.

In order to set your default you need to go to the Video settings. You can select the “never play higher quality videos” option if you generally get low internet speed. But if you wish to see better quality and get good internet speed then you can choose the option “Always Play HD”. This will switch your videos to HD in full screen mode.

There are no manual settings that would play the video in lower resolution in simple mode but will switch it to higher quality as soon as you switch to full screen mode. Also, this is an account specific settings and you have to be logged in with your Google account to change your preferences on it.