A main reason animals exist so large in the ocean is due to the buoyancy of the water. There is no pressure on a blue whale's body because it is floating. However, if you put an animal the size of a blue whale on land the gravity will cause serious problems with it's legs. Some of the largest prehistoric animals were aquatic for this reason. read more
Nor were the ancient relatives of whales that you see pictured on this tree — such as Pakicetus. Hippos likely evolved from a group of anthracotheres about 15 million years ago, the first whales evolved over 50 million years ago, and the ancestor of both these groups was terrestrial. These first whales, such as Pakicetus, were typical land animals. read more