Killer whales are much larger than southern elephant seals. The smallest baleen whale is the pygmy right whale. The smallest cetacean is the harbor porpoise. The brain of the bottlenose dolphin is considerably larger than the human brain. After all, the bottlenose dolphin can weigh several hundred pounds.
Big animals have big brains.
The killer whale probably got its common name because it kills other whales, dolphins, seals, and sea lions for food. Size probably does affect swimming ability. Small animals are more maneuverable than big ones. Dolphins may be able to stay out of the water for hours if they are kept wet and cool. They don't have sweat glands and have to keep their bodies in cool water to lose excess heat.