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