You might think that the largest puppy would be the first-born, but puppy size doesn't always correlate with birth order. Often the biggest is born the last or the smallest is born first. Dog breed is the biggest determinant of litter size. In general, large dogs have larger litters because, biologically, they can safely carry more puppies.
I haven't found anything about it, except when I read it in The Art of Raising a Dog by the Monks of New Skete. They said that first-born puppies are the healthiest and that later in birth order they are more susceptible to health problems. I Googled it and found nothing. Is that silly (like many of the other suggestions in the book?).
The age of the male will affect the size of a litter, but much less than the age of the female. When a male dog ages, its sperm count and quality will decrease. Larger litters will most likely occur when the male dog is less than 5 years old, since it is easier for sperm to enter the female's eggs and most sperm are of higher quality. The total MPI score was used as a response variable and labor, sex ratio, litter size and birth trimester (January-March, April-June, July-September, and October-December) were used as predictors.
I think that the timing of reproduction also matters in the factors that affect the size of the litter, as well as the case of the absorption or abortion of some fetuses by the mother and I would not know if the litter size can really be hereditary, I would like you to research more on this. Humans have control over certain aspects of when and how a dog is raised that can affect litter size, but there are limits. Let's say that for your breed the average litter size is between 8 and 12 puppies, some breeders want to help their mother reach 11, 12 or even 13 puppies. One possible interpretation of this would be that it might be possible to affect the adult behavior of dogs by adjusting litter size and times of birth, but current data do not allow this conclusion.
Behavior was recorded continuously for all litters from birth to three weeks of age with a surveillance camera (Sony SNCRZ25N PTZ with IR LED for night vision), and the video files were stored offline for later scoring and analysis. For example, spaniels and retrievers usually have between four and eight puppies at a time, while smaller dogs, such as terriers, can only have two, maybe three in a litter. In this study, records and observations of 22 litters of German shepherd dogs bred for work in the Swedish Armed Forces (SAF) were analyzed to assess variations in maternal care and their effects on behavior and temperament of puppies approximately one and a half years of age. These results clearly demonstrate that maternal care in the early stages of dogs' lives affects the behavior and temperament of offspring over an extended period of life.
No effect of parity, sex ratio, or trimester of birth was found on the MPI score in this study, although all three were previously found to affect behavior20. You can't put two parents who produced large litters before and expect the puppies to also produce large litters two years later. The results show that females differed consistently in their level of maternal care, which significantly affected the adult behavior of the offspring, mainly with respect to behaviors classified as Physical and Social Commitment, as well as Aggression. Although the results are relative, it clearly shows that the more inbred the puppies are, the lower the puppy count.
Also having mating at the end of the fruiting period of the female dog and not from the beginning is a factor that leads to having more puppies. .