I seem to remember reading that double yolks depend on the age of the hen. I'm not sure that the egg is any bigger though.
I know I get them several times a year...and sometimes more than one in a box, so they aren't uncommon.
btw hellywelly and barmaid are correct in that its usually the very young hens that double lay. But so do many old hens - for eg the battery hens that are rescued.
Its similar to a human - in the beginning of puberty ovulation is not regular and in the peri-menopause neither, so more eggs may be released by the ovary.