Game fills only one quarter of the screen, why? (Android)

Hello,

I have a small snake game project that I'm having trouble with.

When I port my game to Android and play it on my HTC Sensation XL it works fine, fills the screen etc., but on my brother's HTC One M8 it only fills one quarter of the screen no matter what settings I try to use. My friends HTC Sensation fills it properly too.

The M8 is a new model, using Android KitKat 4.4.2. Could that be part of the problem? I wish I had more phones to test the game on.

The resolutions in all these phones are different. HTC M8 has a resolution of 1920x1080, HTC Sensation XL has a resolution of 480 x 800, and Sensation 540 x 960.

So my question is how to set up the screen so that it fills it completely on every phone?

EDIT: Okay, the problem was that the GUI-script was not on the main camera. It still didn't work as well as I wanted, so I made my buttons as meshes in the editor.