Chances are the nucs you purchased contained bees that were not the offspring of the mated queens which were in them. Those queens probably mated with drones that were predisposed to smaller bees and now that her brood is emerged and starting to fly, you are seeing the smaller bees. Some believe that smaller is better and that the larger commercial bees are the result of beekeepers monkeying around with the gene pool. I would not be concerned as long as the bees are healthy. There will be significant variation of both size and coloring within the same hive as a well mated queen will produce bees with anywhere from 16 -25 different fathers. Baby bees will have their thorax covered in hair. As they age the thorax gets more bald. You will notice that the foragers will often have a small shiny spot while the nurse bees do not. There may be other reasons your bees are small, this is just one possibility.
