They are overfed goldfish. Too much fat in them from improper diet. Seen it in all fish species, including common goldfish. You should see the common carps in UK and Germany, they are so fat from overfeeding by people.
if it is a result of improper diet then it is ignorance.
if it is a result of improper diet to purposely achieve abnormally obese fish then it's cruel
if you like it, I don't care, but own up to it at least. admit that it is what you do and we can end the discussion here