Kirk McElhearn:

But there was one comment that I noticed, which I actually found surprising. Tim Cook said, at one point, that:

the number of people who had an iPhone prior to the iPhone 6 and 6 Plus announcements, so this was in September of ’14, that have not yet upgraded to a 6, 6 Plus, 6s, or 6s Plus is now 60 percent. So another way to think of that is, 40 percent have, 60 percent have not.

60% of people are using an iPhone prior to the iPhone 6. In other words, 60% of people are using smaller iPhones.

That may mean that the 4-inch form factor remains the most popular iPhone size of all time. I’m in no position to speak for Apple, but that’s not something I’d let go of too easily.