There could be a few different things going on. The basic size of an egg the hen will lay is inherited. Some breeds were developed to lay large eggs, some medium, and some breeds lay small eggs. But even within the breed, each hen is an individual. Some are going to lay eggs smaller than the breed average, some larger. A lot of that can depend on the flock they come from. If the person selecting which breeders to keep doesn’t select the breeders for egg size, you can get some pretty big swings on egg size and they may not follow the breed tendency after a few generations.
When a pullet starts laying, she normally lays a fairly small egg compared to what it will eventually be. As she lays the egg gets larger, some fairly quickly, some pretty slowly.
A really noticeable change in egg size comes after an adult molt. The first adult molt caused the most change, but there is an increase in size after each one. That’s why commercial operations often force a molt not that long after their flock starts laying, to get the egg size up to a more profitable size.
Diet makes a difference too. Of course a hen needs a balanced diet to lay well, but the higher her protein intake the larger her egg, within reason. I suggest you don’t go overboard on the protein because a larger egg than her body is made to handle can cause medical problems, but a higher protein diet can and will affect the egg size.
According to Henderson’s Breed Chart:
http://www.sagehenfarmlodi.com/chooks/chooks.html
Australorp are supposed to lay an average sized egg. Mine normally lay a bit larger than average
Orpington should lay average to above average. Mine tend to a little under average
Wyandotte should be above average. I don’t have those.
I don’t have a lot of each breed. You don’t either. We’d have to have enough for the averages to mean much and we don’t. It’s that individual between hens.
