Wodering if it was related to oxygen levels in the atmosphere or something else?
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It’s less about the oxygen levels, and more about the CO2 levels. High CO2 levels and warm temperatures made for plenty of plant life, so larger bodies had enough food to consume to maintain larger bodies. Given enough food, a larger body is advantageous for defensive purposes (and also mating, if you can fight off other suitors) so it gets selected.
Some non-avian dinosaurs were big (most notably several sauropod species), but the majority were actually fairly comparable in size to modern terrestrial animals.
Dinosaurian dimensions had little to do with the atmospheric chemistry of the Mesozoic (in fact atmospheric oxygen concentration for most of the Mesozoic was either equal to or less than it is today) and more to do with their unique anatomy. Dinosaurs for example, have hollow (or pnuematisised) bones. Mammals by contrast have mostly solid bones. Hollow bones are strong, without being heavy. This is what allowed some sauropods to reach prodigious sizes that terrestrial mammals likely couldn’t reach (in other words, dinosaurs could get bigger, without weighing as much as a mammal would at the same size). The hollow bones are part of a specialised and highly efficient breathing system in dinosaurs. Besides lungs, dinosaurs also have air sacs that invade the spaces in their bones. The flow of air through this whole system makes for very efficient breathing. It is likely this improved breathing and efficiency which was the initial driver of the dinosaurian hollow bone and respiratory system and evolution later repurposed the trait to facilitate gigantism in some lineages.
Incidentally, this system of hollow bones and air sacs has been retained in modern dinosaurs (birds) where it has been repurposed to aid with flight.
Hope this helps clarify.
High oxygen levels are necessary to allow arthropods to grow to huge sizes, which is why we no longer have bird-sized dragonflies, but vertebrates don’t care as much.
There are a few reasons dinosaurs could get so big compared to animals of today (obligatory “the blue whale is the largest known animal of all time and its alive right now”):
1. Global temperatures and CO2 levels were higher, which allowed for increased plant growth. More plants means more food for larger herbivores. Larger herbivores means the carnivores that prey on them need to get larger too, and larger carnivores means that herbivores need to get even larger to avoid being preyed on.
2. Biological differences between dinosaurs and mammals. For one, dinosaurs have hollow bones like birds do, meaning that giant dinosaurs weighed a lot less than an equivalently-sized land mammal. Dinosaurs also laid eggs rather than giving birth to live young. A modern elephant must be pregnant for nearly two years in order to give birth to a single baby, but a sauropod could lay a clutch of dozens of eggs at a time.
Sauropods we’re pretty much the cows of their day. They lacked the complex system of stomachs that allows modern ruminants to extract decent amounts of nutrition from eating low grade plant matter, so to achieve the same thing they needed an incredibly long gut. A long gut means a large body. A large body needs lots of food to keep it alive, so you have to eat a lot. A long neck allowed a sauropod to graze a wide swathe of vegetation while walking in a straight line which meant they could eat more. The problem with a long neck is that you tip over forward, so add a long tail for counterbalance.
Now you have the basic body plan of a sauropod. Of course they specialised and diversified as life forms are wont to do.
Some scientists have theorized that dinosaurs were so big because of something called the island rule. This rule states that animals tend to get bigger or smaller depending on their size relative to the landmass they live on. So, for example, elephants are much larger than humans because they come from Africa, which is a large continent, while pygmy marmosets are very small because they come from islands off the coast of South America. The same theory could be applied to dinosaurs; if they lived on a large landmass like North America, then they would tend to be larger than other animals. There are other theories as well, such as that dinosaurs may have had high levels of testosterone which made them grow to be large and strong. But ultimately we don’t know for sure why dinosaurs were so big; it’s still a mystery!
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