system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there so many large species that existed in the past such as the megalodon (giant shark) and arthropleura (giant millipede), but not in the present day? Are species tending to become smaller over time?
gpt
Muscle mass requires oxygen.  There was a lot more oxygen at one point.  Scientists don't know for sure if this is directly related, but it's the best theory out there.

So to put it really simple, if there is a ton more oxygen in the air (instead of so much nitrogen, which is what we have), then beasties can breathe much more into their blood and muscle tissue.  So the world develops these really big beasts because it can support them.  

The same goes for oxygen levels in the ocean.

As oxygen levels have decreased over millions of years (it's still pretty high when looking at the planet's history), mammal sizes have decreased since survival of the fittest starts to favor creates that don't require so much oxygen supply to the muscles and blood.