And this particular metric is not a percentage increase in comfort or speed of operations, on the modern phones it can be above/below usability threshold. For example I have a droid phone with a width of 74 mm. But I can't reach with my thumb to the opposite side of the screen, and seems that making phone narrower by 5-6 mm would just about solve this. Thankfully droid OS doesn't require me to do this, but on the other hand I read that in iOS this is a key, most important gesture - "back". And UI button for this operation is in the most unreachable spot on iOS, in the top left corner usually. So users practically forced to do a gesture which is hard to do on a bigger phone.

They are wasted by the mobile UI anyway. /s