system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are animals and fauna no longer as large as they once were? What has changed about our world that mega fauna and mega animals no longer exist?
gpt
We don’t *actually* know but some theories are that temperature of the earth was the major factor for mammals getting so big. If the ambient temperature is lower, the heat/energy of a larger mammal is much easier.  

For insects (and maybe plants), there was a much higher concentrations of oxygen in the atmosphere that helped produce gigantic bugs (we think) and when birds were getting bigger and preying on them (we think) they started to die off (we think)