The Pescatarian Pakicetus
The story of every type of whale started in what is now known as Pakistan some 50 million years ago. In fact all of the cetaceans - the collective family name for whales, dolphins, and porpoises - started their pre-evolved existence there: all as one single mammal, the Pakicetus.
Being a fish lover, the dog sized Pakicetus was a true pescatarian, and just like a labrador, you just couldn’t keep them out of the water. It took millions of years for life to crawl out of the wet stuff, only for the contrary creature to jump straight back in and ask for their flippers back: and yes, this may have taken a considerable amount of time to happen, but happen it did, and their perfectly fine land legs evolved back into sea legs.
Once they were fully aquatic animals again, they were no longer subjected to the gravitational restrictions of land mammals and were able to grow to gigantic proportions; and some of them literally did, i.e., the evolution of the Blue Whale, which grew 10,000 times the size of a Pakicetus.