I have a 1977 Seville with 42k miles that I purchased this summer. I was reading a period 'product information' (A GM publication) booklet of misc specifications for the 1977 model Cadillacs and was SHOCKED to learn that the much smaller 77 Seville (@ 4173 pounds) was 17 pounds HEAVIER than a 'Full size' 77 Coupe Deville (4156 pounds) ! "Burp"
Anomalies like that occurred frequently during GM's first downsizing era. Another example is the '77 Oldsmobile line: a typically-equipped Cutlass weighed 4100 pounds, the same as a Ninety-Eight. When the Seville was introduced during the '75 model year, it was about 1,000 pounds lighter than a '75-'76 Coupe de Ville.