Sea creatures don't need to support their own weight.
Though I'm anything but expert on the topic, my understanding is that many dinosaurs were surprisingly light-weight for their size, and masses of many dinosaurs have been revised downward. As ancestors of birds, whose own bodies are generally of very low mass, and incorporate interesting features (hollow bones and internal air sacs) this isn't entirely surprising.
Though I'm anything but expert on the topic, my understanding is that many dinosaurs were surprisingly light-weight for their size, and masses of many dinosaurs have been revised downward. As ancestors of birds, whose own bodies are generally of very low mass, and incorporate interesting features (hollow bones and internal air sacs) this isn't entirely surprising.