The largest animal ever to exist is alive right now, not extinct

Blue whales can grow past 29 metres and weigh close to 200 tonnes — bigger than any dinosaur known from the fossil record. Their hearts alone can weigh as much as a small car, and a newborn calf already outweighs a fully grown elephant. They feed almost entirely on tiny shrimp-like krill.

— NOAA Fisheries, Blue whale — Species profile