I believe it's a mix of bigger phones and smaller/lighter notebooks.
You've got encroachment on one side from devices with better portability (i.e., pocketable) yet still very usable, the ability to make [cell] calls, a better camera - and on the other side full featured OS on a device that's barely less portable than a tablet with equivalent battery life and a real keyboard (if a keyboard is factored into a tablet there's trivial portability difference).
Plus now, the iPhone is the Watch symbiotic device, again cutting the tablet out of a whole market segment.
Yes, since I have the 6+, I am not using my iPad mini as much as I used to do. There is a handful apps that are iPad only, but most other stuff I do now on the 6+ and I take the mini (which is still very portable) not to as many places anymore since I always have the 6+ with me.