What specifically makes the late Dart Sports/Dusters so much heavier?

Ditto the doors, my homeboy put a '72 door on his '74, and it's a featherweight by comparison.

That said, the later cars have, I've heard, thinner sheetmetal (which is supposedly what makes them rust out so much more than the older ones) in an effort to keep from being total porkers. (This is also the motive behind the smaller fuel tanks)

Seems the best combo for weight would be feather duster parts with earlier doors and bumpers on a later car.