system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why were dinosaurs so much larger than animals today?

Was it because there was a lot more oxygen in the atmosphere?
gpt
One of the theories was due to the higher levels of carbon dioxide in the air. Plants in carbon rich environments grow very well but actually have less nutrients then if they grew in an environment similar to what we have today. So animals needed to eat a lot of plants just to get enough energy to survive. They developed larger stomachs to handle more food and grew bigger to carry a bigger stomach.  

Some Sauropods maxed out the food eating stat and are basically walking stomachs, they have a long neck so they can eat everything in front of them in a large swath without having to walk anywhere. Moving that much mass requires a lot of energy so they stand still and swing their necks around eating everything they can reach. When all the food is gone they just take a few steps and now they can reach a ton of new food. 

With plant eating dinosaurs getting bigger, the carnivores needed to get bigger to be able to take them down. You needed to be big to kill a giant dinosaur, but if you did, there was more than enough meet in one kill to sustain a heard of carnivores. 

End result, everything keeps getting bigger and bigger.