On the iPhone, Apple Music music videos go full screen when I turn the phone horizontal but on the iPad the videos are still in a little window in the middle of the app. Is there any way to adjust this or is that just the way it is on the iPad?
If it's a 16x9 aspect ratio video - it will play letterboxed, you can switch to a zoomed screen by tapping the box with arrows in it in the upper right corner of the video window, under the battery when the timeline is visible.