The egg on the left is double the size of the normal eggs on the right! (Wouldn't you know it was a DKB import hen with my prized DKB 3501 #30.)
Inconsistent transient times of the egg passing through the oviduct may cause abnormally sized eggs due to deposition of differing amounts of albumen (egg white). So some hens consistently produce larger or smaller eggs than the standard size.
A very slow passage time may however, allow for double ovulation resulting in an extra large double-yolked eggs as seen in the above photo. This problem is usually self-limiting and involves only one egg in the clutch and results in the next day's egg laying being skipped. Update: she did skip the next day and laid a normal sized egg today.
Normal chicks can hatch from some abnormal eggs but hatching from a double-yoked one is extremely rare. If you have hatched a double-yolked egg, please post to this blog.