This is probably because the ones in the pet shot aren't getting a good, gut-loaded diet and probably are getting poor exposure to UVB.. not to mention an inadequate water source :/
I bought my female veiled from a local pet shop when she was 2 months old and they had just gotten her and a male in from a breeder. They were the same age, same size, siblings. 3 months later I went in to get some crickets when my order was running behind and the male was still there.. the same one (I asked the owner) and he was still the same size as they were when i bought the female. She had since at least doubled in size. The male had MBD. I bought him too, and he still hasn't caught up in size despite now being a healthy 10 month old.
The age that the pet store gave you was probably correct. Size isn't always the best indicator, especially in an enviroment where they aren't getting what they need to grow and thrive.
