Of course it's big, it's supposed to be big! Funny you say it's impossible to use one-handed, because I often text one-handed with the 6+ and with reachability, it makes it a lot easier to one-hand this phone. Who are we kidding though, if you buy a 5.5" phone, one-handed usage obviously doesn't rank high on your priority list. I'm fine with fact that it's easier to two-hand this phone, and it wouldn't be any different it were a 5.5" Android phone.
Also, the 6+ fits fine in both my suit trouser and slim jeans pockets. You must have very small pockets. Never felt like I've been near to dropping it either, but I've always had a case on it - I know the back can be quite slippery so that's probably why.
The 6 (4.7") is definitely easier to use one-handed, but that's just like saying the Z3C is easier to use one-handed over the Z3, it's obvious. Coming from a Nexus 5, I can honestly say the larger area hasn't really changed how I use my phone.
I've only held a G3 in a phone store, but I do agree it's a better fit in the hand than the 6+. I was looking at one to replace my N5, but ultimately I think there's a few compromises that I'd have to wait until the G4 to solve.
At the end of the day, I think it's a bit silly to make sweeping generalisations on a phone that you've spent 5 minutes using. Experiences are different based on the person using the phone - if you were to say "No possible way for me to use it one-handed..." then that's your opinion - and that's fine, however to post it implying that it's a fact is just plain wrong.