I believe the Full Screen option only applies to letting apps use, well, the full screen (hiding notification and navigation bar and expanding if needed due to the aspect ratio of the phone). However, video is still handled by the app. If it supports it, you'll see an 'aspect ratio' button when you tap the video which should allow you to Fit to Screen, Crop to Fit (that's the option that fills the screen completely). Some apps don't show the icon and you need to pinch in/out to zoom (like YouTube).