http://www.iflscience.com/plants-and-animals/why-monstrously-large-insects-and-spiders-dont-exist

Something to keep in mind when you play FO and see the radscorpions, radroach, etc

Indeed, they cant get enough oxygen for themselves. Why there were huge insects just after plants began to flourish.

Not that we know what oxygen levels are like in the FO setting, post war. (Though I can't imagine it to be more than it is now)

I've known this about this. I resigned long ago to the realization that Fallout runs on 50s science fiction logic.

Nevermind, upon reading this I apparently did not know that. I assumed it was referring to the physical reasons why you don't see giant insects, such as exoskeletons becoming too big to support themselves or the square cube law making it impossible for a giant arthropod to breathe enough to sustain itself with its primitive respiratory system.

While we're at that, the flying ability of Cazadores and Bloatflies is similarly impossible. They'd be far too heavy to fly like that.

Perhaps it's best to disregard them as the same species they once were. A new form a life where the genetic and biological systems are not only different but alien entirely. It would probably baffle scientists trying to understand and connect them biologically.