At least for me it is.
I was perfectly happy with the old iPhone 3GS screen size (3.5"), for a phone - you can easily reach everything with your thumb, and texting is a breeze. Then, for a tablet, I think the 8" is just about perfect... but that means I have two devices. Most of the time that's not a problem, but an iPad Mini can only fit in some of my cargo pants pockets and not others.
So now I'm wondering - do I want to go to a ginormous phone, and just forgo the tablet altogether? When I've played with friends' big Android phones, they seem too big for a phone while being smaller than optimal for things like web browsing. BUT carrying only one "good enough" device has a definite appeal. So the decision is, is it enough of an advantage for me to willingly compromise on the size equation at both ends of the spectrum?
So it's a complicated question - and a potentially expensive experiment.