Dolphin vs. Porpoise. Dolphins and porpoises are cetaceans — marine mammals — that are closely related to whales. Dolphins belong to the Delphinidae family and are up to 30 feet long. Porpoises, belonging to the Phocoenidae family, are smaller and stouter, with other anatomical differences. Though both animals are very intelligent, dolphins are deemed to be more social and live in larger, more stable groups.