In a coupe, the rear chassis stiffening is found in the cross bars behind the rear seat. If the car is equipped with a pass-through rear seat (And it's a coupe) then the bars are slightly heavier gauge, and a rounded opening is created.
In a liftback, the chassis stiffening has to travel to the back, to the C pillar and then hidden (read: Heavier gauge) into the body-side C pillar (Versus the hatch-side pillar supports). There is twice as much steel, twice as much length, and twice the gauge. Then, the glass is twice as much as well.
It's all about the rear chassis stiffening. Coupes will always weigh less than liftbacks (Remember, kids... Long hatchbacks are called Liftbacks. Hatchbacks are stubby cars like the Civic hatch).
-> Steve