Rene Ritchie, writing for iMore:

It sounds great in theory, but if you take an iPhone 6 as thick as the iPhone 4 and start to imagine how heavy it would be, you can start to see where it breaks down.

Apple was deliberate when they pointed out the iPhone 6 was actually lighter than the iPhone 4. They did that because, while thinness is nice and certainly improves the feel of the phone in both the hand and the pocket, it’s lightness that matters. Lightness improves usability.

Rene also notes a few interesting facts about batteries like their propensity to hold heat and their lack of radio transparency.