Fish don't grow to their space, they become stunted from poor water quality in a small space. Just like if you stuck a great dane puppy in a small kennel its body wouldn't stop growing because of the small space, but it might not grow to full potential on a poor diet. I'm not concerned about size either. Fish release ammonia through normal gill function and digestion and goldfish have a large bioload. A big tank is best to keep water conditions close to ideal. And there is a difference between normal veins you can see and those enlarged by stress. I recently treated a fantail for stress and the veining is no longer visible.