Hi,

Your goldfish could be fat, but it seems also to fit the description of dropsy, which is a type of fish disease. A goldfish with dropsy will have protruding scales and appear fat, but in reality it's not fat at all but suffering from bloating due to organ failure. Have you checked the water parameters of the pond recently, such as ammonia and nitrates level.  If the levels are off , it could cause a lot of health problems for the fish in the pond.

If you can take a picture of the fish and attach the picture to your post , it would help a lot in diagnosing the problem. Some goldfish also can get pretty fat so hopefully yours is just that and not anything serious.