
Eggs are released over around 24 hours (ovulation) and are not ripe to be fertilised for another two days.
After that they will only sty viable for about two days so not enough to have any real difference in maturity at birth.
What makes for different size puppies or ones that seem poorly developed is the efficiency of the individual placenta.
Some bitches will allow mating long before eggs can be fertilised, but most bitches will not allow mating after the eggs are no longer viable, hence the old advice of mating a bitch until she no longer wishes it. Some stud dogs will also not mate a bitch until she is on the point of going over, so you may only get one mating or maybe two.
Some pairs will happily mate from the LH surge (about 2 days before ovulation) through ovulation and then the actual fertile stage so possible just over a week.
When I travelled abroad with my bitch she was mated on her 14th, 16th and and 18th day, but she whelped 65 days from the first mating (bitches reliably whelp +/- 62 days from ovulation), so probably ovulated on day 15/16, and didn't actually conceive until day 17 - 19.
A few weeks ago I tried to mate her for a second litter and the stud would not mate her days 14 - 18, showing every sign he didn't think she was ready (she was keen from day 16). We didn't get a mating as he was not available after that.