The issue of size in land mammals, dinosaurs, and marine reptiles is more complex than a simple comparison. While the largest land mammals rivaled or even surpassed the largest known members of dinosaur groups, except for the sauropods, the Mesozoic marine reptiles achieved similar size and mass as modern whales across most major groups, with the exception of the rorqual lineage.

It is important to note that the upper limits of size in these groups were generally comparable, with two specific clades breaking through the physiological barriers that constrained further evolution of larger sizes. The precise mechanisms by which these two clades accomplished this feat are not yet fully comprehended.

For rorqual whales, their ability to attain such colossal proportions may be attributed to the abundance and accessibility of krill, a highly nutritious food source, as well as their remarkably efficient feeding method. These factors likely facilitated their growth to such remarkable sizes.

In the case of sauropod dinosaurs, various factors have been proposed as potential contributors to their immense size. These include their efficient respiratory system based on air sacs, the presence of hollow bones, potential higher atmospheric oxygen levels during their era, and their mesothermic/gigantothermic metabolism. However, none of these factors alone provide a fully convincing explanation as the primary cause for their extraordinary size.

Overall, the evolution of gigantic proportions in these specific clades remains an intriguing topic that continues to be studied and investigated in order to unravel the underlying mechanisms that allowed them to surpass the size limits observed in most other groups.