Dinosaurs seemed to have generally been so much larger than animals today. Huge dragonflies that dwarf their modern counterparts, turtles 10ft long. What is the mechanism that allowed them to be so large, or conversely makes modern ones smaller? Is it about Oxygen levels, or efficiency, or something else?
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Oxygen levels is apparently a myth with maybe the exception of insects. They breathe using spiracles, essentially holes in their body or limbs. It’s a much less effective way of breathing than having lungs and puts a limit on their maximum size. So more oxygen in the atmosphere does potentially increases that allowed limit.
However for dinosaurs that isn’t the case. There were long periods of time when dinosaurs existed where oxygen levels were what they are today or even lower. From what I remember increasing in size was an evolutionary adaptation that worked for them and their bodies became increasingly specialised to “go large”. It was a wildly successful strategy and they dominated the ecosystem for hundreds of millions of years but it also made them especially vulnerable if the climate changed and they suddenly lost access to plentiful food and water. Such as when a large asteroid hits …
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