it's the culmination of small stuff ! airbags, 3-4 computers, power everything and the wiring to support it,bigger wheels and tires, ac, cushy interior, insulation through out, big 4 wheel disc brakes and on and on. Had a similiar thought when I scaled my f-150 supercrew (5500lbs), that started me think about where the weight comes from considering the amount of plastic, composites and aluminum used today.