There are many theories attempting to explain why dinosaurs were so large, including that size related to the massive amounts of vegetation available as food, that size was a method of self-defense and that size resulted from cold-bloodedness. A 2013 study suggests that dinosaurs grew large because of their joints.
The first theory states that dinosaur size was directly proportional to the amount of food available. The Mesozoic Era was characterized by massive amounts of vegetation due to the warm climate and high carbon dioxide content in the atmosphere. The dinosaurs had a large quantity of food to eat and evolved into giants. The carnivorous dinosaurs evolved into giants as a result of the larger herbivores.
Another theory states that the size of dinosaurs evolved as a form of self-defense. Massive plant eaters were less likely to be killed by a predator, regardless of how slow their large size made them.
The third theory suggests that dinosaurs were large as a result of being cold-blooded. Scientists believe that extremely large dinosaurs could maintain their internal temperature regardless of external influences. However, scientists are not certain that all dinosaurs were cold-blooded. Some paleontologists believe that the Tyrannosaurus Rex was in fact a warm-blooded carnivore.