I did a restoration on a 71 Hemi Cuda some time ago. It had a trashed drivers door so we found a replacement from a 73 or 74. That replacement looked identical, but it must have weighed 50 or 60 pounds more. It was due to internal bracing for side impact protection I would guess. It was amazing the difference in just a few model years.
New cars are made from lighter materials, but they have to add so much structure to meet crash standards and so many options, they never seem to get any lighter overall.
Check out the link for a comparison of a 2010 Camaro vs a 1968.
2010 Camaro SS vs 1968 Camaro SS - Tampa Bay sports car | Examiner.com
The 2010 is about 800lbs heavier than the 1968!