I am not a small guy but I can't reach the whole screen with a single thumb like I could on the 5s. And the problem is that iOS places all sort of important commands at the top of the screen (back button, URL bar, etc), in the unreachable area. So it means the 6s can't be used with a single hand which is not a small UX change.
I hope they will make smaller future versions, or move all the essential functions to the bottom of the screen. Right now even the icons on the desktop are top aligned.