In reading about the new iPhone 6s coming out, I found it odd that it has a smaller battery than the iPhone 6 did. Given that battery life issues are one of the top concerns for iPhone users, this seemed very strange.
I did the math, and the battery size on the iPhone 6 (1810 mAh), plus the battery of the iPhone 6s (1715 mAh), plus the battery of the Apple Watch (205 mAh) still doesn't equal the battery on my phone (Droid Turbo, 3900 mAh). Battery life isn't all about battery size, but that's certainly a major part of it.
My tradeoff, of course, is that my phone is a bit thicker by a whopping 3.7mm. I'll take it. The problem with iPhones in this regard is that Jony Ive, the Chief Design Officer at Apple, has made it very clear that battery life is not a top concern. His feeling is that if the phones were a bit thicker to accommodate a larger battery, users would find it less "compelling". My anecdotal evidence against that is Wednesday nights helping in the sound booth for the middle schoolers, and how it's lined with iPhones that need to be plugged in because they couldn't make it through a single day...
They seem to be doing the same with the Apple Watch. They only put a 205 mAh battery in it to help keep it thin, but as a result the screen can't stay on and you need to turn your wrist in order to activate it. Not a big deal, but an odd shortcoming for a watch. As a contrast, the new Moto 360 watch will have a 400 mAh battery. In this case the Apple Watch is 0.9mm thinner, but the Moto 360 is arguable a much better looking watch anyhow, and it has double the battery! -- Photo: http://i-cdn.phonearena.com/images/articles/174516-image/Custom-Moto-360-designs-weve-made-with-Moto-Maker.jpg
I expect this problem will be solved in the coming years as battery technology improves, but right now it's interesting. Would you allow your iPhone to be 3.7mm thicker if the battery would last twice as long?