There's two reasons it weighs more which I think are relatively secure to state:
1/ The LTE radio
2/ The bigger battery required in order to support LTE
and a third possibility -- that Retina Display screen sure sucks power, I'm guessing.
There's only a hour difference in quoted battery life between LTE and WiFi so I doubt that's the reason. A brighter backlight for the Retina display seems like the most obvious answer.