Ed Yong in The Atlantic:
The first time I came face to face with a sea lion, I nearly screamed. I was snorkeling, and after a long time spent staring down at colorful corals, I looked up to see a gigantic bull, a couple of feet in front of my mask. Its eyes were opalescent. Its long canines hinted at its close evolutionary ties to land-based predators like bears and dogs. And most unnervingly of all, it was huge.
Mammals tend to get that way when they invade the ocean. The pinnipeds—seals, sea lions, and walruses—tend to be immense blobs of muscle and blubber. The same could be said for manatees and dugongs. And whales are almost synonymous with bigness. Time and again, lineages of furry mammals have gone for a swim and over evolutionary time, they’ve ballooned in size. Why?
Most of the explanations for this trend treat the ocean as a kind of release. The water partly frees mammalian bodies from the yoke of gravity, allowing them to evolve heavy bodies that they couldn’t possibly support on land. The water unshackles them from the constraints of territory, giving them massive areas over which to forage. The water liberates them from the slim pickings of a land-based diet and offer them vast swarms of plankton, crustaceans, and fish to gorge upon.
But William Gearty from Stanford University has a very different explanation. To him, the ocean makes mammals big not because it relieves them of limits, but because it imposes new ones.