The size of the egg has an influence on the time the embryo needs to grow to fill the shell and with that on the hatch time. Bigger eggs need more total incubation time, as at the same egg shell temperature the embryo will grow at more or less the same rate, but as there are more grams of egg and grams of final chick, it will take longer to hatch.
A rule of thumb is that with an increase of the average egg weight of 1 gram, the total incubation time is extended with approximately 30 to 40 minutes. So when the average egg weight of a flock increases from 65 to 70 g, we need to set the eggs approximately 2.5 hours earlier if we want to pull the chicks at the same moment.
There are some factors that can influence this relation between egg weight and incubation time.
Young breeder flocks (broiler breeders younger than approximately 32 weeks) produce small eggs that are hatching relatively late. This has to do with the fact that the embryos in these eggs at moment of lay are slightly behind in development (more in the pre-gastrula stage than in the gastrula stage) and need more time to complete the incubation process.
Often bigger eggs are getting warmer during incubation, as the size of the eggs block the air flow over the eggs, and with that limited air flow the heat transfer is less and the eggs are getting warmer. Due to this higher temperature the embryos will develop relatively fast and the total incubation time might increase less than the mentioned 30-40 minutes per gram of egg.