ELI5 Dinosaurs - why were they so big?

Why were dinosaurs and things that lived long ago so much bigger than animals today? What caused the extreme weather back then? Were oxygen levels really much higher than they are today. Were CO2 levels higher too? How are both possible?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: vinneh, the_original_retro, thechromaticscale, xeno_man, davehone, cloudnyne, mayumoogy, mgroz83 and zefrogleaps

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