We'd speculate the thicker handset is down to the addition of Force Touch technology in the new iPhone display.
Thick and fast
Force Touch first debuted on the Apple Watch and allows for different pressure on the surface to do different things – it'll mean a harder press on an app would have a different functionality to a quick, light tap.
Force Touch hasn't been confirmed for the phone but the rumours all point toward the technology being introduced as it was used within the Apple Watch as well.
The thicker handset is also posing a big problem for case manufacturers who believed the design would be the same as the iPhone 6 and it may mean you're not able to use your case from last year's phone.