Can't say for sure, but as I got it the xapk is a kind of a raw apk grid. It has the actual icon of the apk, but when you open it you can't play. Instead you have to, in-app, once again download the necessary additional data for being able to play the game. And that data can be more than the data amount you downloaded from the app-store for that game. One may correct me if I'm wrong, but I understand it like that.