What's interesting is, the question isn't really "how did dinosaurs get so big?" The biggest non-sauropods (Shantungosaurus, Lambeosaurus) were only a little bit bigger than Indricotherium, Deinotherium, and the biggest Mammuthus species. The question is really "how did sauropods get so big?" Everything else seems to top out at 15-25 tons (rhinos (biggest Indricotheres) and theropods (biggest Spinosaurs) ~15, proboscideans 15-20, hadrosaurs 20-25...), but sauropods reached at least 80-100 tons (Argentinosaurus), maybe more (Amphicoelias fragillimus and Bruhathkayosaurus, if they're as big as estimated, are mindboggling, but based on little evidence).