Full screen mode is not always the full screen mode, especially on your smartphone. The larger the screens and the narrower the bezels, the more phones have an aspect ratio that does not match most YouTube videos. Fortunately, the video sharing app offers a sophisticated workaround to fill your screen.
First, open the YouTube app for iPhone or Android and search for a video to play. This trick does not work on YouTube in your mobile web browser, so you'll need to use the official app. Next, play the video and turn your phone into landscape mode. Although the video is technically full screen, there will most likely be black bars on the sides of the video.
To make the YouTube video fill the entire screen, simply press with two fingers. At the top is the "Zoom to Fill" balloon, which confirms that your video is now filling the screen of your phone. Press the button with two fingers to return to the original video size.