Dinosaur anatomy was supremely well adapted for gigantism. Pneumatized bones with air spaces and more efficient respiratory systems.

The reptiles of today with sprawling legs existed back then and they were small back then as well. You can't compare iguanas to dinosaurs

There is a lot less air now which caused us to be smaller is what I read. Everything was bigger

@@mp2764 that's not true. Where did you read this?

They were just big because there was a lot of food and they were good at being big. Ornithischians were very similar to mammals in bone structure and food ingestion, but egg laying allowed them to survive extinction better than mammals even when big, so you had a lot of large ornithischians. Sauropods, unlike mammals and ornithischians, did not have to chew their greens allowing them to ingest more greens, thus they got huge. Theropods had so much giant sauropods and large ornithischians to eat they could get huge for predators.

The answer is always food, it also explains why theropods had small arms as a way to not accidentally hurt each other during a food frenzy and got infections.

@@elnovillomapuchedehomerus2412That’s not actually why we think they had small arms. We think their arms grew relatively small to allow them to grow their heads much larger and still keep their bodies balanced - balance would have been incredibly important for large predators like theropods, and prioritizing their heads meant they could focus on evolving those into efficient weapons and not overweigh their front halves through shrinking their arms.

Oxygen levels doesn't actually do that, not in the way you think. Sure, It was the case for insects, but it's because they have a weird way of respiration. Their size is limited to the amount of oxygen they can get. Dinosaurs were a different story. At the time, the oxygen was lower than today's. The reason why they were so big is mostly 2 reasons: respiration and reproduction. Dinosaurs had the most efficient lung system in all animals, the avian respiratory system. They have air sacs in their bones, allowing for more sustained energy, and at the same time, it allowed them to be relatively lightweight for their size, enabling them to get really big. Theropods, Pterosaurs, and Sauropds have this respiratory system as well as birds. Reproduction in dinosaurs were fast as they can have a large cluster at once, allowing for more variety and increased survival of offspring.

There wasn't more oxygen in the Mesozoic period. One of the reasons that sauropods could grow so big is due to their birdlike lungs which allowed them to breathe in more oxygen.

@@jonathankennedy1963 Hollow bones may have also been a factor, hollow bones are more structurally stronger than bones full of marrow. Another very important factor is feeding method. Sauropoda, unlike ornithischians, elephants and hornless rhinos, do not need to chew their greens allowing for more caloric intake.

@@trvth1s I did mention the bones, yeah. And yes, I forgot to mention the efficient digestion of sauropods.

​@@jonathankennedy1963Worth pointing out that oxygen levels probably isn't even the case for gigantism in arthropods, as there are times that land/flying arthropods were around without high oxygen levels (the largest _Arthropleura_ specimen was found in rocks before the oxygen levels in the Carboniferous skyrocketed, and it as well giant griffinflies were initially still around during the early Permian).