Smartphone displays are rarely the same aspect ratio as YouTube videos. That leaves black bars on the sides. You can pinch-to-zoom to fill the screen, but if you’re always doing that, you might as well make it default.
Here’s the situation: You’re watching a video in “full screen” on your phone and you see black bars on the sides as shown below. It’s easy to make the video fit the entire screen by pinching your fingers out like you’re zooming in.
Now the video fills the entire screen—though it does cut off some of the top and bottom. This is a good solution, but you have to do it every single time. YouTube actually allows you to make this the default way videos are played in fullscreen.
Select “Settings” from the menu.
Now go to the “General” section.
Toggle the switch on for “Zoom to Fill Screen.”
That’s it! You will no longer need to zoom in on every YouTube video to take up the entire screen. Again, this does cut off part of the video, so keep that in mind. But now you can use all the screen real estate of your device.