Wikipedia on Sauropds. The graphic showing relative size may be of particular interest; Argentinosaurus is in robins-egg blue. (Amphicoelias, the big maroon silhouette, is a form known only from one enormous vertebra; all assumptions about its length, size, and weight are considered highly speculative.)
The blue whale article includes a similar size comparison, this time with the human scuba diving. The disparity in body shapes will be evident. (Nobody has a problem with the idea that an elephant will way more than an anaconda, even though the snake may be longer; the same general principle will apply.)
What about Dimetrodon in the Permian? I always had the impression it was larger than the amphibians it preyed on…although amphibians tend to be carnivores or insectivores, come to think of it. I’m not sure what the largest contemporary herbivore was.
The first tetrapod herbivores evolved about 300 million years ago. The first tetrapods evolved about 395 million years ago. The largest terrestrial tetrapods during those early years would therefore have necessarily been carnivores.
Edaphosaurus was a contemporary of Dimetrodon. There also would have been a brief overlap between Dimetrodon and the large pareiasaurs near the end of Dimetrodon’s time, during which the latter would likely have exceeded Dimetrodon’s size by a fair margin.
