Try using
Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("vnd.youtube://" + video_id);
startActivity(intent);
The reason is the different Uri. The one you are currently using is just supplying content via http: that happens to be video and get's resolved to youtube. The one with "vnd.youtube" is actually telling the system that you have video content you would like one of the native apps to take care of.
Ahh, if you want to actually play full screen video without using the youtube app (which you can not control) you don't you try to just make your own VideoView?
Check out this link
playback video full screen