If the movies are full width on the screen, it’s just a matter of the aspect ratio of the movie not being the same as your TV. But there are settings you can play with.
I’ll describe this on the Shield, but it should apply generally to all devices:
- While a movie is playing, press ‘down’ on the remote.
- Go to the action menu (far right of the control bar, 3 vertical dots)
- Go to Playback Settings.
The bottom item on this menu is “Display Mode.” Play with this setting to find what works for you. Once you do this, I believe it should apply to all movies played on that device, so you should only have to do it once.