Wodering if it was related to oxygen levels in the atmosphere or something else?
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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.
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