So what is the increase in weight attributed to? If you guess the new Force Touch display, you'd be right. Apple has added a new capacitive sensor layer to the display.
Designed to measure microscopic changes in pressure, capacitive sensors are integrated into the backlight of the display. These measurements are then combined with signals from the touch sensor and accelerometer to provide fast, accurate, and continuous responses using a series of complex algorithms.
The new sensors mean the weight of the display on the iPhone 6s has increased to 29g from 12g on the iPhone 6. Likewise, the weight of the display on the iPhone 6s Plus has increased to 40g from 19g. That's a 2.11x and 2.42x difference.
Check out the weight breakdown of the various components in the iPhone below...