Rene Ritchie on iMore:

Take an iPhone 6 as thick as the iPhone 4 and imagine how heavy it would be. 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, it’s lightness that matters. Lightness is what improves usability.

While I often agree with Rene, I have to disagree regarding lightness—it does matter, but it’s not what improves usability the most.

The idea of a thick phone with longer battery life sounds great precisely until you actually try to hold it up for prolonged periods of time. Then it causes fatigue and eventually prevents you from using it for as long as you’d really like to.

We’re talking about 129 grams in the form of the iPhone 6 here. I actually have a 143 gram HTC One M7 on hand, with an 4.7″ screen and the weight difference is negligible. What really makes the 6 usable is it’s thinness, allowing me to use the phone with one hand. My hands aren’t that big, hence this whole argument varies from person to person, but the 2.4 millimetre difference in thickness plays a much bigger role in ergonomics than it seems it should. I assume—and I admit that this is just a guess, but backed by experience with other thicker and heavier phones—that I could easily sacrifice one extra millimetre for a bigger battery, as well as some additional weight, just to make it last a bit longer.