Unfortunately there is no thorough answer to this question, however many people have attempted to provide possible answers in regards to why whales are so big.
One possible answer is that water is a much denser and colder atmosphere than air and since whales are mammals (warm-blooded) their large size helps keep them warm by providing them with large muscles, more calories to consume (which can be converted into energy and body heat) and thick blubber which helps insulate their body heat in these cold (sometimes polar) environments.
It is also important to know that the larger an animal is the less calories it needs to consume to stay warm.
The large body mass of these whales also helps them displace the frigid temperature of the water and keep it from causing harm to their internal organs.
Simply put the large mass of whales helps to keep them warm in cold climates.
The bigger they are the easier it is to stay warm!
Another answer is that their large size could be an evolutionary adaptation over the course of millions of years that has helped whales survive in cold aquatic environments, defend themselves against predators and hunt for food.
After all whales (or rather their earliest relatives) existed over 50 million years ago so evolutionary survival could be a possible answer in explaining their massive size!
Water provides fewer restrictions than land when it comes to an animals ability to grow, and size can be a good deterrent against predators.
In most cases larger animals are less likely to be attacked and thus are able to survive with fewer threats.
Water also provides buoyancy and a sense of weightlessness which allows whales to move freely without any constraints due to their size or weight.
It would be impossible for a large whale to maintain its body weight on land because the large surface area its body would need to hold up would be too much weight for its legs (if it had legs) to handle and it is likely that the whales internal organs would be crushed by the weight of its body even if it were able to walk on land.
On land mammals have to hold the weight of their body up with the strength of their legs and the muscles in their body need to be able to keep their internal organs in place in a weighted environment.
Land animals are also limited by their mobility, food supply, water supply, available space and numerous other factors.
The larger an animal is the more difficult it would be for these animals to find food, shelter and a stable living space.
In the past when dinosaurs roamed the earth food was much larger in size and much more plentiful since the food dinosaurs hunted were significantly larger in size.
In today’s world however it would be almost impossible for an animal as large as a whale or dinosaur to live on land.
Not only would their large size limit their ability to navigate most land environments, but they would have trouble maintaining a stable food supply.
The blue whale for example can grow to lengths of up to 98 ft and consume as much as 40 million krill per day!
The amount of food a species of that magnitude would need to consume on land would be extremely difficult if not impossible for them to find largely due to the fact that most animals wouldn’t be large enough or plentiful enough to provide them with the nutrients and calories they would need to survive.
It would be difficult if not impossible to find and consume that much prey on a daily basis, and even if a whale could find that much food on land it would quickly become scarce.
Not to mention the fact that a whales throat is too small to swallow most mammals and their digestive system most likely wouldn’t be equipped to digest the food properly anyway.