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The second would be that there is some bias in the collection of animals, in that a larger proportion of the larger animals are easier to find as fossils, and it's easier to find a big fossil than a small one.
The first is that there was a greater percentage of oxygen in the atmosphere ...
There has historically been much more CO2 in our atmosphere than exists today. For example, during the Jurassic Period (200 mya), average CO2 concentrations were about 1800 ppm or about 4.8 times higher than today.
You see, some of the most interesting dinosaur bitts sadly don't fossilize.