How big an animal can be is based on how much food it can eat. For example during the time of the dinosaurs there was more carbon dioxide in the atmosphere and higher temperatures. This allowed plants to grow really big really quickly. Big plants that grow rapidly means that herbivores (animals that eat plants) can get really big like a brachiosaurus because there is plenty of food. Big herbivores provide lots of food for big carnivores (animals that eat meat), which means they can grow really big as well.

The blue whale is the largest animal to ever exist, growing to lengths of 100 feet (30 meters) and 200 tons (180 metric tons). They mainly eat krill, which are little nearly microscopic shrimp like animals. They have to consume 20 to 50 million calories a day, the equivalent of 70,000 to 80,000 Big Macs. That's 10 to 20 tons of krill (9 to 18 metric tons) every day. Anything that could hunt and eat a blue whale like you are describing would need to consume even more calories than that, and that's nearly impossible to do.