Alot of the weight in the centre/rear is the frame...then you add the differential, drive shaft, rear axles, driver & rear seats, gas tank, trunk lid, and etc...
To give you a better idea of the structural weight...look at the rear...then look at the front; majority of the front are panels...(which also includes the super light-weight aluminum hood)
The front still house all the heavy stuff like the engine, battery, transmission, and etc, but engineers put it as far back as possible...relocating the centre of gravity all the way back...help evening out the weight ratio.