On YouTube on my iPhone when I want to enlarge the video I have to turn my phone sideways. Is there any way to enlarge it when upward

On YouTube on my iPhone when I want to enlarge the video I have to turn my phone sideways. Is there any way to enlarge it when upward

Possibly, but only if the original video was recorded in portrait (vertical) mode. Most videos are wider than they are tall. In order to get the largest viewable screen, the phone has to be held horizontally so that the WIDTH of the image/video is maximized. Holding it upright (portrait) would mean the width would be limited, and the height would also be limited in order to maintain aspect ratio.