A few iPhone 6s rumors said that the new models would have smaller batteries than the iPhone 6, even though the phones themselves would be slightly thicker. This didn’t make sense to some people initially, but the news became more logical when coupled with all the Force Touch leaks, which is what Apple’s 3D Touch display was referred to in rumors.
Apple needed to add various components to the iPhone 6s series, so the screen can actually register the different pressures of touches and taps, which lead to an increase in iPhone thickness, but also to a smaller battery capacity.
