system
Given the following post, respond as if you're commenting on Reddit:
human
Why could dinosaur's grow to much larger sizes than modern day animals?

The post about the world's largest ancient snake inspired this question. Why are there no animals today that come close to the size of the largest dinosaurs?
gpt
The blue whale, a current living mammal, is the largest animal that has ever lived. It is substantially larger than any dinosaur we know of.

As to the thrust of your question, why are terrestrial animals smaller today (I assume). Well, that question has a lot of suppositions. I've seen everything from increased availability of plant material (the climate/atmosphere of the time supporting greater growth), less effective defensive strategies from plants, greater need for size to compete against larger predators/prey. It's not necessarily a cut and dry question, unfortunately.