I agree with the runts do not always end up to be the smallest of the litter, and they can be a bit more then the average size pup in the litter with a mellow, good temp

. I would look for the right temp in the pup and the parents. I had pick of a liter only suppose to be 60 lbs and he is 74 lbs (not fat either). My last dog was the runt and he ended up being almost a 100 lbs