Some breeds of chickens lay smaller eggs and some lay larger. Also individual birds varying in laying as well. You may find a hen who lays a larger than average egg consistantly but then you may get one that lays a smaller egg than normal.
Eggs start out small when a pullet is first laying. As she ages and matures the eggs get larger and in general reach a more standard size. You will one occassion find a larger egg, maybe a double yolker, ir the egg factory will send a small one down the shoot but most often the eggs will be about the same consistant size from each mature hen.