What factors affect egg size?

The main factors that influence egg size are genetic and nutritional.  

1. Genetic factors 

Some birds simply lay bigger eggs because of the breed they are. These have been selected to produce larger eggs. To be a successful breed, they also have the ability to lay these large eggs easily. Others lay smaller eggs because they are a smaller sized bird e.g. Bantams 

2. Nutritional factors 

Generally speaking, when a hen who is actively laying eats more calories, she will take the opportunity to produce a larger egg. This is an important underlying factor when investigating prolapses, for example, as if a hen is taking in too many calories, she will try to produce a larger egg than she is built for, which can cause a prolapse. 

Essential nutrients can also influence egg size, for example Methionine (an amino-acid), and Linoleic acid (a fatty acid).