Okay, so this was never my original idea to begin with, but I heard Jason Snell propose this theory on a podcast—can't remember which one, he's on so many—several months back, and I immediately agreed with him. Then, as time rolled on, I had considered writing about it myself. But, luckily my procrastination paid off and Jason finally put his theory in writing and now I can link to his piece (I've been meaning to post this for a couple of weeks), which is excellent.

Snell:

Over the years I’ve said numerous times that when it comes to battery life on iOS devices, Apple appears to have a target battery life in mind and builds its hardware—a balance of power-saving software, hardware efficiency, and battery capacity—to hit that number.

It’s an observation born out of reading spec sheet after spec sheet over the years while writing reviews of new iPhones and iPads. Every year, people who are frustrated with their iPhones running out of juice before the end of the day hold out hope that the next iPhone will ameliorate the issue. In general, those people have not been satisfied.

I think that this is Apple's medium-term strategy. I believe, that just like MacBooks hung in the same range of battery life for several generations before making recent leaps, the same thing will eventually happen with iOS devices. As all the technologies that suck up power in iOS devices—screen, processor, radios—get more power efficient, and battery technology improves ever so slowly, there will eventually come a time when iOS devices can make a similar leap. Apple will likely also get to a point where diminishing returns of making devices thinner will result in less battery space being stripped out of the devices with each new generation.

I've got the iPhone 6 Plus, and the extra volume in the device really does make for much improved battery life, as Jason Snell points out. It's by far the best feature of the bigger iPhone 6. I like the big, beautiful screen, and the camera is supposedly better with optical image stabilization—I'm not good enough to be able to tell—but the biggest advantage of the big one is the battery life. If it wasn't for the battery life, I'd most definitely opt for the smaller form factor of the iPhone 6.

For now, I'm happy with my more-than-full-day battery life of my iPhone 6 Plus, but I definitely look forward to the day when iPhones double or triple their battery life while staying the same size or even thinner than the iPhone 6.