I'm using the Xbox One version of Antstream Arcade and it isn't full screen. Why is this?
At the moment, the Xbox One version of the app needs to be confined to the "safe space" in the middle of the screen so that things don't get lost off the edge on different screen sizes. Modern games are designed so that nothing important happens at the edge of the screen, but the retro games that we provide didn't consider this, and so we have to keep the app confined so that nothing potentially vital is lost.
We are looking into options for having a full screen app in future though, so watch this space!