i would say that the litter size is more to do with timing of the matings, fresh semen meeting ripe eggs and obviously the number of eggs released along with the quality which i am sure can vary quite alot. my girls first litter was 9 a decent size for breed but her second (to same sire too) only 2 very small!
Both my bitches had 2 litters each and the first one had 4 both times and her daughter had 8 both times!! coincidence or not i suppose will never know!!