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There is actually no uncontroversial explanation for the as-of-yet unparalleled size of dinosaurs.
If you look at the distribution of large sizes in dinosaurs, one distinction becomes clear: the only dinos that were small (less than 1m) were carnivorous theropods. Except for the 70 cm Fruitadens Haagarorum, no herbivorous ornithischians were small.
The evolution of teeth in ornithischian lineages proceeded along very similar lines independently, and each innovation was accompanied by a bump in body size. Teeth are merely the most fossilisable sections of the digestive system. If they changed, then the rest of the animals' physiology was also changing in such a way as to enable larger body sizes, from getting larger guts to becoming quadrupedal (the first ornithischians were small and bipedal). In turn, this leads to changing ecological opportunities, feeding on different plants, being able to eat much more at once, and thus beginning the cycle of ever-growing sizes.
This would then enter into a coevolutionary back-and-forth with their theropodan predators, eventually resulting in the sauropods, tyrannosaurids, and the other giants.
What were the ornithischians eating to support their sizes? There has long been an assumption that the high CO2 levels of the time would have driven growth of the plants, as it does with some plants nowadays. This assumption is nowadays being challenged though, with current thought being that the Mesozoic was a time of lower plant productivity.
In the high-productivity case, there would have been so many plants to eat that the dinosaurs would have grown large simply because they had unlimited food.
In the low-productivity case, large body sizes result in lower metabolic rates and longer digestion times. This means that proportionally, less food is needed by a larger herbivore than a smaller herbivore, so large size is an advantage.
Of course, the influence of body size on fitness is so great that there probably isn't one answer that fits all. The plants would have allowed the growth of large sizes to be maintained energetically, but evolutionarily speaking, body size would have been maintained because of positive effects on mortality, speed of sexual maturity, offspring qualities (size, number), and other such factors. Such things are pretty hard to study in extinct organisms.
Importantly, it should be pointed out that while, generally, dinosaurs are orders of magnitude larger than mammals, the only truly gigantic dinosaurs were the sauropods. The ornithischians I talk about above are mostly elephant-sized (except elephants don't have long tails, making them seem smaller).
In other words, only sauropods, like the 30 m long, 100 ton Argentinosaurus, were characterised by gigantism. The smallest sauropods (e.g. Diplodocus) were still the size and weight of two elephants, and even the island-dwarfed sauropods (e.g. Europasaurus) were larger than all but the mammalian megafauna. This makes sauropods both interesting and tricky. Interesting because we can study them to see how gigantic body size evolved, but tricky because the results from their study aren't directly transferable to the rest of the dinosaurs.
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