It’s great that you are questioning what you read. That’s how we discover new things. Mammals can be many sizes. A shrew only weighs as much as a few paperclips. A blue whale is the biggest animal that has ever existed on earth.

Being in water ALLOWS blue whales to get that big, but it doesn’t force them to be that big. Bigger isn’t always better.

Blue whales filter tiny animals called krill out of sea water. They basically swim up to the krill, open wide, and swim right on through. Then they squirt the water out of their mouths and catch the krill on big comb-like things called baleen. Krill can’t dodge and weave to get away. Whales with baleen are called Mysticetes and they tend to be very big. Dolphins are hunters. They have to be fast, smart, and agile to chase their prey, mostly fish. If they were the size of blue whales, they would be slower to move and turn. It would be harder to catch the fast, maneuverable fish. It would be like a tiger trying to catch a mouse.

Sperm whales are the biggest whales with teeth (Odontocetes). They specialize in eating giant squid, so they may need to be big and strong for this.

Odontocete whales tend to hang out in groups. Any idea why they do that?

That's a great question!
Although some whales can get large because the buoyancy of water allows them to have less restriction on mass than land-based animals, not all mammals that live in the ocean get so large, as you pointed out. Whales and dolphins are both in the biological order of cetaceans,and dolphins are actually in the suborder of toothed whales (Odontoceti), Family Delphinidae (Dolphin), which includes both common dolphins, like the one you see off the coast of Santa Barbara, and killer whales (Orcas). So dolphins are a type of whale, just smaller than the bigger whale species!

This difference reflects that different species who have similar life history traits and physiological traits (in this case marine mammals) evolve different, specialized characteristics (e.g. big, slow growing, plankton-feeding baleen whales vs. smaller, faster growing, predatory dolphins) to survive in their environment and coexist with each other. All organisms, both on land and in aquatic environments do this.A familiar example of this is the difference between domestic cats and mountain lions.

Whales are larger than dolphins for the same reason why elephants are larger than mice: the physics of living on land versus in the ocean impose a maximum size that an animal can attain, and they also set a minimum, but between the maximum and the minimum there is quite a range of possible sizes. The actual size of an animal is determined by its ecology, and dolphins have different ecological preferences than whales do.

You asked an interesting question. There are probably many reasons and events that led dolphins and whales to evolve the way they have, but let's consider an argument about obtaining food.

There are two broad categories in which we can place whales: toothed whales vs. baleen whales. Toothed whales typically hunt for their prey, whereas baleen whales feed on prey low on the food chain, and acquire their food in a more passive manner. Let's think about what that means in terms of anatomy: toothed whales that hunt for their food end up being smaller and more agile so they can catch their prey, while baleen whales tend to be larger. That said, let's think about how dolphins acquire their food. Aren't most dolphins active predators, like toothed whales? So wouldn't they also want to be streamlined, and agile? In many cases, dolphins may actually be of comparable size to toothed whales. For the cases where they are not, there are other factors that may also be related to how whales obtain their food vs. dolphins, which bodies of water you can find them in, and so on.