Google has been constantly working towards making its YouTube mobile experience better for users.
Keeping that in mind, the search giant has now started showing YouTube vertical videos properly as a complete full screen for its Android users.
Instead of rotating the video content 90 degrees on tapping the full screen, which is not helpful at all, the updated YouTube app for Android fits videos vertically. With older versions of the app, such as the YouTube v10.25 currently available on google play, vertical videos become only slightly larger in landscape mode.
http://m.gadgets.ndtv.com/apps/news/youtube-for-android-now-shows-vertical-videos-properly-in-full-screen-718828