Also keep in mind that these creatures, for the most part, need food. Animals generally need an amount of available food proportionate to their mass. So a frost worm, weighing 8000 pounds, is going to need around 44 times the amount of food a human eats, meaning that same amount of land supporting 180 humans supports just 4 frost worms. ALSO, the trophic pyramid. Frost worms are predators, meaning they only eat other animals. Animals add up to only 1/10th of the mass of their food supply, so Frost Worms, needing to eat the animals that live off that square mile of land, each need at least 2 square miles of land. Also, they live in really cold places, which aren't known for their abundant life, so that could easily extend to dozens of square miles per worm.
Of course, all this assumes they're the only type of predator around, which in this case isn't that much of an assumption, but in other cases can reduce the viable population further.
Whenever you think there might be too many of a certain creature, use the CR and the food method, and pick whichever one is least.
Oh, and another thing: Dragons. Dragons fuck with that CR thing, since you can't realistically expect that less than 1 in 4000 dragons survive to adulthood. It's ludicrous. What I do is I use the average of the wyrmling CR to the CR of the dragon's age category to calculate commonness. With it, I can calculate that in the entire world there are only a few of each type of Great Wyrm, which checks out as more or less accurate to me.
Also, Dragons don't need much food. I usually just let them live off 1/10th the otherwise required amount of food; it works just as well.