Take the Size Difference with a Grain of Salt
There is a very slight difference in the thickness of the iPhone 6S when compared to the 6. And because it's thicker, just like with human beings, that means it weighs slightly more. However, until I held the 6s in one hand and the 6 in another, I couldn't really tell that there was a difference in the weight or girth of the phone. Take this with a grain of salt then, quite literally, as that's about the difference in thickness between the 2 devices.