What's up with this egg?

[robnms]

I have a Barred Rock laying eggs and and today she laid this big egg, she has been running loose in the yard with a Golden Seasbright banty and 2 days ago one of my Delawares mated with her. Is this because of the size of the chicken or just nature?

 

[bethsplace]

just nature most days they are they same size but sometimes you may get a smaller or larger usually a double yolk if real big the hen is the hen it doesnt matter who she mates with she still lays the same size egg