Is "Display size" option not present on tablets?
Does both of them have same Android OS? Like AOSP or else?
My suspect is the setting based on Official ROM. In this case the Nexus 6 use 'Display Size' to adjust DPI, it adjust font size to match DPI means larger DPI smaller font or as you wish.
In Lenovo you get fixed DPI but you can adjust only font size.
Hope it helps