Which is the largest animal on Earth?

The blue whale is the largest animal of any kind, living or extinct, reaching around thirty metres and a mass beyond a hundred tonnes. Being aquatic is what allows it. A land animal's weight has to be carried by its skeleton, and bone strength scales with cross-sectional area while mass scales with volume, so legs quickly become impossible past a certain size. Water supports a whale's entire bulk, removing the limit. Feeding does the rest. Blue whales lunge into dense swarms of krill, taking in a volume of water larger than their own body, then push it out through fringed baleen plates and swallow what is left behind. Harvesting a tiny, extremely abundant prey species in bulk delivers far more energy per unit of effort than chasing large animals would, and it is the reason the ocean's biggest hunters are filter feeders rather than predators of anything substantial.

A blue whale calf gains weight astonishingly fast on milk with a very high fat content.