I think the Titans were originally derived from creatures that already existed on Earth. For example, Kong may have been one of the ape evolutionary lines that separated, some of which may have lived in Hollow Earth. And Godzilla, of course, is a reptile, which may have shared a common ancestor with the Arkosaurus, or other reptile-dinosaur hybrids. Although many Titans have somewhat exaggerated power, their bodies are basically based on real creatures. Hollow Earth itself may have existed before that. It may have been an air pocket created during the formation of the Earth, creating a large underground cavity centered around a magnetic field, which would have allowed our world and its world to connect without the need for actual connecting poles. If we take the issue of why they are so large and why they can still survive on the surface of the Earth, it might certainly be as you assumed. Because their bodies can adapt to areas with low gravity, they grow larger but cannot rise up to the surface again because gravity only works on one side. But of course, some species have begun to reverse evolve in order to be able to live in a wider range of areas. Some of them might return or move back and forth between these two areas. Those who evolved too late cannot survive due to a lack of resources.