I'm new and i'm french

I follow all the thread about the tronsmart s89 but i have one problem with XBMC on S82 Measy.
I've got a 4k TV and I can put the resolution of the S82 to 4k correctly. I can watch all of my movie with Mxplayer wich uspcale (1080p to 4k) the movie, and the quality is amazing.
But when I triy to use XBMC for watch my movies, the screen take about 1/4 of the TV (the 1080p movie is display in native resolution to the 4k TV and is stuck on the up left corner of the TV).
The weird thing is that the UI of XBMC is display in fullscreen on the TV but not the movie.

The S82 is Root with the last firmware (102k4) and I can't change the resolution of XBMC (stuck in 1080p).
Can you help me please because I don't know how to resolve this.

Thank you !
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