The slight 0.2mm increase in thickness could be the result of Apple adding pressure-sensing Force Touch technology to the next iPhone, enabling the smartphone's display to distinguish between a light tap and firmer press and complete different actions accordingly. The “iPhone 6s” is also rumored to adopt 7000 Series aluminum, which could possibly contribute to marginally different dimensions.
But the new thicker size is probably to little to get rid of the protruding camera though. Even with the new rumored 12MP camera it will probably still protrude this time. And it also probably means that it will have a tiny bit bigger battery that together with the new power saving features in iOS 9 means that the iPhone 6s will have even better battery life than the iPhone 6 has.
It's almost certain that the iPhone 6s will get Force Touch, and It will be very interesting to see all the new innovative ways we can use Force Touch on an iPhone.