There for a while when our Australorps first started laying, we got a lot of double yolkers, one day Sally laid this monster egg that would'nt even come close to fitting in the slot in a large egg carton. Then she did'nt lay for about a month! I'm surprised it did'nt kill her! There was a guy on cnn a while back that had a hen lay a huge monster egg and his hen died later on that night!
When my girls first started laying, the first week someone laid 2 triple yolks and 3 doubles. Even my dear old grandma hadn't seen that! The 3rd yolk was a bit smaller, but intact! Now, just simple plain ole 1 yolk eggs. I guess they had to get it figured out.
Are they new layers? For about the first 3 months 3 of my 8 always laid double yokers and now they have stopped with the doubles but are still good sized like yours.