I have a concern. This concern is a pretty personal matter, I'll be the first to admit. There are other people who have this same concern, but by and large, it appears that, like the classic matter of phones with keyboards, this is a thought shared by a small number of people.
Cell phones are too big. It's no longer just an issue where there is a flagship or two that is huge. Most mainstream phones are available in 5-inch or larger display sizes, and 4.5-inch options (let alone a 4-inch display like the iPhones 5/5C/5S and 3.5-inch displays like the classic iPhones 4/4S and earlier) are increasingly uncommon and reserved for exceedingly low end handsets.
A lot of my friends (and myself to a certain extent) have the same thoughts about keyboard phones, which today exist only in old and low end handsets, most of which are also "small."
In her article on Motherboard, Casey Johnston describes some of the problems of big phones. They're awkward to use, they're nearly impossible to reasonably use with just one hand, and where software-based solutions do exist to try to make them more usable one-handed, the solution is hard to use, not easily discoverable, and is itself problematic.
I think part of the problem is that phones are one of the more obvious status symbols, and so making a phone bigger and more obvious is a good way to create a product that is "about" visibility.
The other aspect to big phones is that they do admittedly allow for more hardware. You can fit and cool a bigger and faster processor, include a bigger battery, more storage, bigger antennas, and so on. In addition, as people start to use cell phones as their primary or only computing device, having bigger options does make sense.
The other thing we're seeing is that as people trend toward more expensive and more capable cell phones, many of which are faster and more capable than most small tablets, we're seeing people trend away from having a laptop, a cell phone, and a tablet, to either a laptop and a cell phone, or just a cell phone. So, with people using them that way, it makes a certain amount of sense to have a really big device as an option.
Another thing I think has influence is that even though the top end iPhone (a 6s+ with 128GB of storage) costs $950, it's pretty easy to split the cost of this device into smaller chunks, with financing through carrier, one of the many new upgrade plans offered by carriers, or financing from the device manufacturer. Apple's price for a 128GB iPhone 6S+ is a few pennies short of $45/mo if you buy it through the iPhone Upgrade Program in one of their stores.
The 32GB (only size) Microsoft Lumia 950XL is $650 and the 128GB Google Nexus 6P is also $650, so Apple's pricing here is a little further out. Nevertheless, it's not uncommon for people to want a cell phone that costs $500 or more, which is starting to be lot for a device whose lifecycle should be about three years, but which will end up needing additional warranty or insurance coverage, and may suffer a pre-mature death, simply because it's so easy to drop or misplace a cell phone these days.
I currently use an iPhone 5S (which has a 4-inch screen) in Apple's leather case. I keep the case on it because it adds a good amount of grippable texture. The problem with anything bigger than this is that it's too thin to hold onto solidly, and too big to wrap your hand around. In addition to being unable to use it one-handedly, you can barely even carry modern big phones in one hand, and they often feel more fragile as well.
Plus, driving the biggest displays and most power hungry CPUs and LTE radios on the go appears to be a problem for cell phone batteries, which creates range anxiety, a problem that didn't seem to exist back when phones were about 3-4 inches in size. Battery banks and battery cases are becoming common sights in stores and the idea of range anxiety related to just about anything that has some kind of battery is pretty common in society today. From the iPhone 6 and 6S' battery problems that were apparently significant enough for Apple to build a battery case for the devices.
Interestingly, Apple got panned for the fact that the case only holds about half what an iPhone 6/6S does internally, and for the case's odd shape. I think the idea was that Apple identified a need for a little more range, but not so much more that they needed to build a much larger case. I also don't think it would have made sense for Apple to build a case that gave the iPhone 6S a few days of run time, because most people don't have a realistic need for that. I think that for the people who have that need, outboard packs are the preferred way to get it. Many big phones (including the Lumia 950XL and the Nexus 6p) throw around battery capacities as an advertised feature now, instead of just talking about how the battery is sufficient to run the phone for a day or so.
I don't spend a whole lot of time watching other people use their phones, so perhaps I'm misunderstanding something core to this transition to bigger phones. Maybe people are using them less, or because they're substituting a large expensive phone for a phone and a tablet, they're just using them with two hands, or maybe some innovative applications have sprung up around devices that have really huge displays. (I can see how the Lumia 950XL's giant display would make a great viewfinder for the 20-megapixel camera that shoots DNG files, for example.)
Something that you can technically do, but I almost never see people doing, is connecting a USB or Bluetooth keyboard to most phones today (the iPhone will only work with Bluetooth, but Windows 10 Phones and Android phones can work as USB hosts for keyboards.) The biggest problem with that is, what is realistically possible on a phone where a keyboard is reasonably necessary? Are people doing this or is this just an excuse somebody with really big hands makes to other people? I've tried using a Bluetooth keyboard with an 8-inch Windows tablet for writing, and wasn't very impressed with the experience.
Given that the experience is not likely to be very good, I do wonder if there's some contingent of people trying to do "productivity" tasks on big phones set up this way (with Bluetooth keyboards, possibly even mice, depending on the particular system.) To me, it seems like these phones would be worse for voice calls, due to their really large size, so you'd think their primary audience would end up being people using them to do other things.
I hear frequently from people that bigger phones are easier to type on and the screens are (now) usually better, and I can imagine they are for some things.
I think a big question is, in the case where a six-inch display sitting on a table is actually good enough for writing or whatever other productivity task, is how does phone hardware handle being used for actual productive workflows? What happens to my Word document if I open it in Word on my iPhone 6S+ or my Lumia 950XL and then switch over to the web browser to gather a link? Traditionally, phone hardware isn't powerful enough to keep this all in memory at once, but a lot of phones now have 3 or more gigabytes of RAM.
It's not very hard to see why big phones are eating the lunch of small tablets, but I think it's still harder to see them displacing bigger tablets. I think another real problem here is that we're entering an age where what people want is a tablet that's always connected, but tablet data allocations don't match up with what people will use on them. I suspect a lot of people would buy devices that shifted the balance toward being tablets first and voice communicators second, but perhaps this is just because I want to be able to port my phone number to my iPad or Surface and use a headset or Bluetooth device with it for voice calls. HTC previously built a Bluetooth handset for just this purpose, and doing this would reduce cable clutter in bags.
The other neat thing is that some vendors (HP) appear to get the potential of Continuum. The Elite X3 and its docking stations should make for a better continuum experience than the Lumias 950 do. Not good enough and there are still other problems, but for somebody doing simple tasks on the go, it could be a lot better than tethering a phone to a regular computer, and if documents are open on your phone, connecting your phone to whatever display happens to be available and most convenient at the time makes sense.
I'd like to try a big phone at some point. I'm skeptical that they're really a very good experience in terms of mobile phones as we expect them to behave today, but perhaps this is just me failing to keep up with the times. While I'd like to try one, I have a hard enough time keeping a 4-inch phone like the Lumia 521 or the iPhone 5S in my hands, so I'm excited for the possibility that Apple might introduce a true 4-inch successor to the iPhone 5S.