I agree. Apple has to be looking at 40 million sales of the Galaxy Note line (all high-end devices) and wondering if they should make a play for some of that. Sure, we all mocked it, and Samsung wasn't even the first with a 5" Android smartphone (Dell beat them by a year with the Streak but no one bought it), but evidently there is a not-so-small market out there, particularly in China, which Apple claims is so important to its growth.
I would not be surprised at all if they introduce a larger product alongside an iPhone 6 with the same or slightly larger screen size. Given the direction of the ridiculous screen wars on the Android side, the "small" screen of the iPhone would actually be seen for the major strength it is if Apple also had a larger hybrid device. I know a LOT of people who complain that phone are getting too big, and who like the iPhone because it is still pocketable. When the 4.7" Moto X or HTC One screens pass for "small" in the Android world, Apple has a clear differentiator with the 4" screen in a premium product. Android manufacturers dumb down their 4" products.
However, they'd need to make a choice as to whether it runs the iPhone or iPad version of iOS, or if they "fragment" iOS further. Unlike Android, iOS is not resolution independent. That's not necessarily a bad thing. Resolution independence is why there are so few native tablet versions of Android apps. Developers make one version and expect it to run on all Android devices. A "big" phone app or "little" tablet app doesn't look right on the tablet or phone, respectively. That's probably why the Android market has gravitated to big phones and small tablets while Apple makes small phones and big tablets (the iPad mini is considerably bigger than 7" Android tablets and has a different aspect ratio).
That's probably why they haven't done it yet. They are probably unsatisfied with how both the iPhone and iPad versions of iOS look on a 6" screen. Not wanting to repeat the Maps debacle they are probably content to take their time.