Due to Bendgate, Apple went ahead with 7000 series aluminum with increase in thickness but that hasn't made the new iPhone heavier. Contradicting earlier rumors claims that iPhone 6S will be heavier due to newest aluminum series is the report from The Verge who have uncovered Apple's environmental reports on the device.
According to the chart, the 3D Touch display on iPhone 6S weighs 29 grams while the 6S Plus weighs 40 grams. That's almost twice than the multi-touch found on iPhone 6 and 6S Plus with 12 grams and 19 grams.
As the new technology of 3D Touch is just born, no doubt that in coming years, maybe in iPhone 7, designers at Apple will be bringing down its weight.