YouTube displays videos with different aspect ratios based on the platform and video format. The YouTube video player automatically adapts to the size of each individual video.
Note: This feature is available only on mobile devices.
The YouTube app for iPhone and iPad automatically adapts the player to the size of the video. Whether it’s vertical, square, or horizontal, the video will fill the screen. The vertical (portrait) video player matches the aspect ratio of the video - getting taller for square and vertical videos, and shorter for widescreen videos.
How vertical videos are displayed
Vertical videos will now display in the video player without black bars on the sides. For example:
A viewer can tap 
