So this can happen at unusual resolution scales on some devices. You can try adjusting your device screen resolution and seeing if different resolutions better fill the screen in game. This may also just be a bug with the layout on your specific device in which case if the adjustments to resolution do not solve the issue, I recommend reporting the issue on the bug report forums.
Make sure to include the device make/model, OS version and what screen resolution you have the device set to.
On the mobile client you cannot adjust your resolution, the only options are to change the FPS, the music and sfx volumes, language, screen shake and spactate. Then the 2 buttons for misc (credits) and privacy.