Here's a fascinating little Youtube link about a huge chicken egg.
Apparently this guy got an enormous egg from his backyard hens. He wondered how many yolks it might have, so he had his wife videotape opening it.
Over six ounces -- getting up toward half a pound.
Cracking the egg...
Now here's a surprise! A fully-formed egg within an egg!
I'm so glad this was video-taped, or I wouldn't have believed it.