Hey, @TeamYouTube, why on earth would you make the back button on Android minimise the video? Yknow. A feature you get when you just swipe down the video. Now if I accidentally click on a video, I can't get back to the one I wanted to watch -- the purpose of the _back button_.
We recently made an update to the back button–now it takes you back to the watch page. A second back button tap will minimize the video player and a final tap will exit the app completely. You can share feedback about the change directly from YouTube: https://goo.gl/zcddRh