Full screen is not always full screen, especially on smartphones. As the screen gets taller, the bezels become slimmer, and the aspect ratio of more phones doesn’t match most YouTube videos. Fortunately, video sharing applications have a nice workaround to fill the screen.
First, open the YouTube app for iPhone or Android and find the video you want to play. This trick does not work on YouTube on a mobile web browser, so you must use the official app.
Next, play the video and rotate the phone to landscape mode. Although technically the video is full screen, black bars are likely to appear on both sides of the video.
Now, to make a YouTube video fill the entire screen, just pinch it with two fingers.
A “zoom in to fill” bubble will appear at the top, confirming that your video is now filling the phone screen. Pinch with two fingers to return to the original video size.