@Weightaholic: Believable in the Fallout Universe's standards.
As long as it's set on Earth, it's not merely implausible, but unbelievable. Way too large.
http://www.youtube.com/watch?v=v9_WxFKLSio
http://www.youtube.com/watch?v=KsbFgxo9Im8
All about the air man, all about the air.
http://www.youtube.com/watch?v=KsbFgxo9Im8
All about the air man, all about the air.
And here's why... it's not primarily about the air. It's also about the gravity. The way creatures with exoskeletons grow larger is by shedding their exoskeletons, and forming a new chitinous exoskeleton which hardens. Until it hardens though, there is no support for the internal structures/organs, thus leading to the collapse of the body of the insect if they get too large. Basically, they collapse under their own weight.
Ever eaten soft-shell crabs? Why are they soft-shelled?
At least people have actually learned something factual in this thread now....