no to tiny dolphins
It turns out there is a fundamental reason why we can have tiny fish, but cannot have tiny dolphins. Recent analysis shows that mammals balloon in size as they transition to a completely marine lifestyle. There is a small range of perfect body sizes for living in the ocean when you are a mammal, and it seems to be right around 500 kilograms (1,100 pounds). This magic number is all about heat and metabolism. The ocean is cold, and mammals have to retain heat to survive. How much heat you can make is determined partly by how many cells you have, so a tiny dolphin could never eat enough to generate sufficient heat to replace what it loses to the water. On the other hand, if they get too much bigger than 500 kilograms, it is difficult to find enough calories to maintain their size without making some major feeding adaptations, which is how whales manage to tip those scales. So that is why evolution says no to tiny dolphins.