Photos not using full screen via AirPlay?

I’ve used Mylio via Air Play using Apple TV physical device and Apple TV app (Samsung) and in both cases the photos don’t display to the full size of the TV screen. The same photos, when sent via AirPlay from the Photos app, make use of the full screen.

Is there a setting that I’m missing, or doesn’t Mylio know how to utilise the full real estate of the screen?