You really don't have to change their diet until the last two to three weeks of their pregnancy.... then switch to a good quality puppy food.
The size of your girl does not determine how many pups she will have... although the bigger they are the more likely they are to have MORE pups. It all depends on whether the mating took place at the appropriate time and how many "eggs are in her basket" (if you get my drift).
All the best to you. I also have pups due the end of September and another litter the middle of October.
