Egg sizes/weights

Young hens start off with smaller eggs and they do get gradually bigger.  They can be slightly smaller in the winter and larger again once they are able to eat for longer during the day.  It takes over 24 hours to produce an egg so they lay later each day and then miss a day out.  Then their next egg is sometimes larger than normal. 

Some of the really large ones can be double yolkers as 2 yolks are released at the same time and end up in one egg instead of 2.  It is better for the hen not to have too big an egg but I don't think there is anything you can do to prevent it.