[Marko413]

"Ancient arthropods were able to grow to ginormous sizes for two main reasons. First, oxygen was much more concentrated in the atmosphere 400 to 500 million years ago, making up 30% of Earth's atmosphere instead of 21% today. Arthropods rely on air flowing through their bodies in order to take in oxygen; they don't have lungs like us. The extra oxygen basically served as added fuel for growth. Second, and more simply, there weren't really any larger predators around to cull their numbers."

So what I am hearing then is that the 21% oxygen in our atmosphere is a good thing. Otherwise spiders will grow to enormous sizes.

[C19fan]

Actually yes. That is why you had biggest terrestrial bugs that ever lived were around during the later Carboniferous period.