"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
iPhone 6s and 6s Plus Weigh More Primarily Due to 3D Touch Display
Apple published Environmental Reports for the iPhone 6s and iPhone 6s Plus that reveal the devices are about 11% heavier than their predecessors, as first noted by The Verge. But the additional weight comes almost entirely from the 3D Touch display, which weighs nearly twice as much as a regular ion-strengthened display.
The material breakdowns for the iPhone 6s and iPhone 6s Plus list the display weighing 29 grams and 40 grams respectively, while the iPhone 6 and iPhone 6 Plus displays weigh 12 grams and 19 grams respectively.
Meanwhile, the individual weights of the aluminum, battery, stainless steel, glass, circuit boards, plastic and other materials on the iPhone 6s and iPhone 6s Plus are very similar to the iPhone 6 and iPhone 6 Plus.
iPhone 6s and iPhone 6s Plus weigh 143 grams (5.04 ounces) and 192 grams (6.77 ounces) respectively, while the iPhone 6 and iPhone 6 Plus weigh 129 grams (4.55 ounces) and 172 grams (6.07 ounces) respectively.