Now this is an example of out of the box thinking from those crafty engineers at Motorola. There's a new software feature on the Moto Z and Moto Z Force. It's called "swipe to shrink screen" and it's sole purpose is to allow easier one-handed use for folks with smaller hands.
Above is a video from a YouTube blogger (Erica Griffin) showing off this handy idea (at the 9:53 mark). The way it works is that you just have to drag your finger upwards from the bottom of the display. This is different than previous one-handed modes which typically have you move your finger to the left or right side. The starting point is in the center-bottom on the display, and touching the black area exits the mode. Nifty!