Bug facts within the comic! 

Since bugs, put simply, breathe through their skin (and no, I'm not talking about Quiet from Metal Gear Solid 5), and most can't stock air since they have no lungs, having more oxygen to breathe was a big deal back in the Paleozoic era for bugs. They could reach a bigger size! 

And then there was also the fact that dinosaurs came by at one point and found bugs made a pretty good lunch so that must not have helped :)