Interestingly enough arthropods cant achieve the size they have obtained in the fallout universe. An insects size is proportional to the amount of oxygen in the air. With probably half of the worlds oxygen producing plants either destroyed or dying. I would actually predict all non aquatic exoskeleton organisms shrinking in size, regardless of FEV or radiation mutations.
Yes, and gravity has nothing to do with it...
Giant Radscorpions are the devil. You can run into them between Goodsprings and Primm, and when you're just starting out you figure, "oh, everything levels with me, right?" And then you unload all your ammo into it while running backwards and it kills you the first time it's tail hits you.
So then you realize, oh, Obsidian makes more realistic games, and sneak around them until level 10, and decide "time to take on those giant rad scorpions." Only, once again, Obsidian makes realistic games, and your 9mm and Varmit Rifle are just as useless as they were before. So now you go down in a few hits instead of one.