As we already know, the new iPhones are slightly heavier than their predecessors. From 6 to 6s, we went from 129 to 143 grams; from 6 Plus to 6s Plus, from 172 to 192 grams. We are talking, a, of a difference of about + 11%.
Right away, one could imagine that this extra weight comes from the reinforced housing by Apple to eliminate #bendgate once and for all. But that had already been eliminated from the equation, after all, the aluminum structure (now of the 7000 series, like the Apple Watch Sport) itself is even slightly lighter than the previous one.
This chart explains the thing well:
As we can see, the “culprit” of everything the new screen 3D Touch. The components involved in building it including the new pressure sensors and a stronger glass of ons-X now weigh more than twice as much as before, from 12 to 29 grams in the case of the iPhone 6s and from 19 to 40 grams in the case of the iPhone 6s Plus.
No wonder that Apple didn't even mention it, in the keynote
(via The Verge, iClarified)