How come some goldfish have fat stomachs?

Their are many varieties of goldfish, however goldfish with fat or potbellied stomachs are classified into the "Egg Bodied" goldfish category. Egg Bodied goldfish are their own distinct group of fancy goldfish which all have an unusually large abdomen. The most popular goldfish in this group are the Orandas, Ranchus, lionheads, and the pearl scale/Hama Nishiki.