Christopher Mims from the Wall Street Journal wrote this:

Given the size of its current battery, a little back-of-the-envelope math reveals it’s more than reasonable that if the iPhone 6 were as thick as the iPhone 4, the iPhone 6 could have double its current battery life.

Rene Ritchie, writing for iMore responded:

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.

Read the rest of Rene’s post. It’s spot on.