This may very well be dropsy in your goldfish , tell me how long has the fish been like this if its a long time then we can rule out it being egg bound and even dropsy ,
Why because the fish may very well be this fish is carrying a tumour.
Have you gently felt the stomach if not could you to see if there is a hard lump inside, if yes and there is a hard lump then your fish has a tumour
Dont worry its not the end of the world because though unsightly fish can live with tumours for many years so dont for the moment think you've already lost this fish we have a koi (shusui) that has been with us for 5 years now that developed one which we monitor from week to week by looking at it .
Should it start to grown rapidlyand thus start to be problems and the fish cannot cope with them then we would choose to euthenate the fish with MS222 .
Then again in saying all this it may well be that your goldfish is a bit of a pig and simply eats more than other fish (it does happen).
We recall a thread about a group of fish with swollen stomachs, it turned out aftr questioning the poster that his mother was responsible for feeding whilst he was aways resulting in some very overfed goldfish looking like this either earlier in the year or perhaps last year I cant remember exactly when .
Dave