Runts Can Surprise You Sometimes...
Sometimes the runts will surprise you and turn into a monster when more mature.
Of course, other times they will just always be a smaller dog.
It’s a roll of the dice.
6 pounds at 6 1/2 weeks is light for sure but not totally out of the range. How is their poo? Any diarrhea? How many times per day do they eat? What diet are they on? Do the other puppies let this dog eat? Is the breeder making sure they all get good food, or do they just drop a plate and run?
These are bigger questions, especially considering you have already decided on this dog.