It is difficult to give an average weight since there can be a wide varience in overall type and size in the American Bulldog breed as a whole. I have seen full grown adults weigh as little as 45 pounds and as much as 140 pounds.... I always say if the dog appears to be healthy and sound, then the weight is spot on for them.
As far as runts go, I have seen runts in litters mature to be the largest in the litter. Runts are merely pups that were conceieved later then their siblings (dogs can breed several times in a heat cycle and sperm can stay viable in the female for several days after a mating, so certain pups can be concieved 5 or so days later than others), and while as pups, they may be smaller, they catch up.
The best way to get an idea of where your pups could be at is to speak with the breeder or people who own other siblings if at all possible. Otherwise, don't worry about weight in the least... if your pup appears to be healthy, active, etc. just see where time takes it!
The average for our 8 week old bulldog pups is between 8 and 10 lbs, so 8lbs for yours isn't unheard of. Plus, once you take them home and they no longer have to compete with their siblings for food they usually gain pretty quick!