The usable space on a screen doesn’t grow much between a 4" and a 6" phone, because you hold your thumb further over the screen, and more parts are unreachable.
And while one could move the content into the top-left or top-right (depending on handedness of the user), the result is so ugly users preferred a UI with less real estate over it in tests.
With a 4" phone, the best reachable areas are the top and the left. Which is why Android 4.0 – which was right around the time when 4-5" phones became popular – moved all these actions into the top and left edges with the Action Bar and Navigation Drawer.
With a 6" phone, those areas are unreachable, the bottom and the right are obscured by your hand, and the only area left for controls is the center – but you can’t really break up content around the controls either.
The only solution that works partially are gestures, but those are very limited in what you can do, and are very inaccurate.
If you have a better solution for a chat client in which a user might have 400+ channels open than using a navigation drawer (with search) on the left, content in the middle, and input at the bottom, then please present some.
But currently, only for a small handful of apps the 6" problem is solved, and I believe that for many apps, it might never be solved.