How did dinosaurs get so big?

There was an inherent selection pressure in favor of increased body size for many species. Because you must share your evolutionary niche with other members of your species, being slightly larger than them allows you to eat higher leaves, catch larger animals, and defend a broader region. More significantly, the larger you are, the less vulnerable you are to predators.

This selection pressure has led many animal lineages to steadily grow in size over time. Scientists in South Africa stated in 2009 that dinosaurs became bigger as they transitioned from walking on two to four legs. However, being enormous means you require more food, are less able to avoid natural calamities, and reproduce more slowly. As a result, the fossil record is replete with instances of species that gradually grew in size before becoming extinct.