It does happen from time to time and is not uncommon and does not indicate that anything is wrong with her. Just check her bum to make sure it didn't cause a prolapse. The egg is most likely a double yolker. Maybe she's an over-achiever.
we get those too from our chickens I am not sure which hens lays them our Road island red's,or the barred Rock hens . I have also seen tiny eggs a while back when our hen started laying and our duck layed one too the big brown egg is normal size ..
The largest chicken egg I could find online was from China and weighed 201 grams. After working on the conversion, it was about 7.09 ounces. I hope you contacted Guiness on that one!