Question about weight

Why is the 1970 Dodge Dart Swinger the lightest of all the Valiant-Dart-Swinger-Scamp species?
The second lightest being the 1969?
The difference is about 300 pounds between these two years and its lighter by 4-500 pounds over ( or under, so to speak ) than some other years.
This is, of course, for the same body and the same engine-trans combo.

Where did all the lard go?