A small tank could cause stunted growth, however an axolotl would not be in a pet store for 2 years. The animal turn over in pet stores is pretty fast, especially for something as unusual as an axolotl, which is one reason they can "get away with" keeping animals in substandard conditions. They only stay briefly. Unfortunately some owners will duplicate the conditions in the pet store instead of researching the specific environment needed for their species.
Chances are very likely that the people in the pet store have no idea how old the axolotl is.
My guess is that your axolotl is actually much younger, like mine and Kaini's, and not stunted BECAUSE you found it in a pet store. My reasoning is that pet stores buy in bulk, especially smaller quickly selling animals like axolotls. They probably buy them from a local breeder as well, because, while prolific, axolotls are still uncommon and shipping them is a pain. A breeder would not hold on to an axolotl for 2 years before selling it, especially one that will easily reach 5 inches in a few months even in less than ideal conditions. Also a breeder would not sell a defected or stunted animal to a pet store because to them it is a business and why ruin your reputation for an animal you probably got $10 for?
Since you only recently obtained your axolotl, and I am guessing it is still a juvenile since it came from a pet store, I would continue to feed it daily, and only cut back if you notice the belly is getting fatter than the width of the head.
