most of the times it is just cause the fish are not yet bought, i mean have u seen the pet store bags they are tiny, and they dont use large bags often, just for the large pleco's, koi's, and other large freshwater fish. every now and then they will buy a very large one just so when people are looking to buy that type they are not all the same sizes, some people just want to speed up the process of fish growth and buy a already big fish. All and all it is both, just depends on the pet store, and how big the store is. if its a big store they dont need to relie on buying big fish for business, if its a small pet store they may buy larger fish for better business and something different.