Several factors contribute to the differences in size between fish and marine mammals. Firstly, fish have been present on Earth for over half a billion years, whereas marine mammals, such as whales and pinnipeds, are relatively young, with an evolutionary history of around 50 million and 30 million years, respectively. Fish have had significantly more time to diversify and occupy various ecological niches in the marine environment, including both miniaturization and gigantism. Pinnipeds, being more recent in evolution, still split their time between land and sea, which limits their upper size due to the constraints of terrestrial locomotion.

The minimum size of marine mammals is likely influenced by the need to conserve heat and the ability to move efficiently. Water conducts heat much faster than air, and marine mammals rely on insulation to retain body heat. The risk of hypothermia is significant, even in relatively warm water, as water conducts heat up to 25 times faster than air. The square-cube law, which states that the volume of an object increases faster than its surface area as it grows, plays a role in this. Larger animals have an advantage in staying warm in water but face challenges in dissipating heat on land. This principle also applies to the size limitations of terrestrial animals.

Furthermore, the distance that marine mammals can move and their respiratory needs affect their size. All marine mammals require air and must come to the surface to breathe. Lung capacity and movement rate increase with body size. The ability to hold one’s breath and dive for extended periods is essential for larger marine mammals. Sperm whales, for example, can dive to great depths and hold their breath for up to an hour and a half, while dolphins can hold their breath for 15 to 20 minutes. Body size is often associated with swimming speed, allowing larger animals to cover greater distances. In contrast, smaller marine mammals would have limited diving capabilities and would need to remain near the surface. The largest whales have no natural predators, while smaller marine mammals would be vulnerable to predation by larger fish.

The evolution of Ichthyosaurs, ancient marine reptiles that lived alongside dinosaurs, offers an intriguing comparison to whales. Some species of Ichthyosaurs reached immense sizes, up to 70 feet, while smaller species were just over a foot long. Although Ichthyosaurs were reptiles, it is unclear whether they were strictly cold-blooded. Certain pelagic fish, such as tuna, can regulate their body temperature above that of the surrounding water and remain submerged indefinitely. Ichthyosaurs may have possessed intermediate traits between whales and tuna, allowing them to stay submerged for longer periods and attain smaller body sizes compared to whales.

In summary, the size differences between fish and marine mammals are influenced by evolutionary history, thermal constraints, locomotion abilities, and ecological factors. Fish, with their ancient lineage and extensive diversification, have occupied a wide range of sizes and niches in the marine ecosystem, while marine mammals have adapted to specific thermal and locomotion challenges, resulting in their characteristic sizes.