The biggest different with other goldfish is that my japanese ranchu never had problem with overfeeding. Never in my past experience although I put in alot of food, they seem to know their limit. It is opposite with other goldfish which are always hungry, never stop eating until they overfeed and get float upside down.

Maybe the japanese breedline have better anatomy to the others.
Hope comment from expert on goldfish breedline specialist here.

Cheers,