Hi Dominik,
the references to other libs than sap.m do not hurt. If reloading the page after starting phone simulation does not help, I guess there is some element in the page that has a width or min-width set.
If you don't immediately know which one it could be, you can go through the HTML hierarchy in the browser's F12 tool and check the sizes of the elements. I'd start around that big circle thing.
Regards
Andreas