How quickly do Threshers grow? [minor spoilers]

The ultimate challenge in Borderlands 2 is Terramorphous, a gigantic thresher. We see some other gigantic threshers around the place, too. In the Pre-Sequel, we learn that threshers (two tiny ones) were introduced to Pandora by Sir Hammerlock, which therefore must have happened between BL1 and BL2 - a five-year period.

So the obvious implication is that Terramorphous is the same as "Terry" from the end of that quest. That means he's had maybe three years to become absolutely ginormous. Is that really how it works? I can't find any information anywhere about the biology of these creatures; probably few people have survived to report on this (other than vault hunters, who are too busy picking over the orange loot).