I believe litter size cannot be contributed to just one thing, but rather the given size of any litter is based on many variables such as heredity, the individual, the health and condition of the individual at the time of conception, timing of the breeding, the condition male's sperm, nutrition during pregnancy, stress during pregnancy, etc etc etc.
From my personal experience, I can tell you these things regarding litter size:
1)My bitches, what every their litter size, are generaly close to the same every litter given that the male's sperm is always in optimum condition adn the timing of the breeding is right. For example, I had one bitch that threw between 10 to 12 pups every litter, while others threw just 6 or 7 every litter. The only times I had a bitch that threw a significantly different number of pups in different litters was due to the timing of the breeding. For example, one bitch's average litter size was around 9 pups, but one time she had only 3 pups, and this was because she was bred early in her cycle and just once during the cycle.
2) If you decide to breed a bitch in her later years you will probably see litter size decline in those later years. And, as with people, some bitch's go into "menopause" early. For example, when I was growing up, my mother bred a female once a year whose average litter size was 12 to 14 pups. Her last two litters, however, when she was 7 and 8 years old there were only 4 pups each. Likewise, a few years ago, I had a bitch whose average litter size was 7, and her reproductive cycle declined early. She only had 4 pups in her last two litters as well, but she was only 3 and 4 years of age at the time.
3) Finally, while I have had offspring of my bithes thow the same litter size as their dams, I have also had a few that threw very different litter sizes than their dams. Same thing goes with sisters, and other relatives. So you can't always go by family in terms of litter size, but it may be a good way to make an educated guess.
