I've never owned a goldfish but I own a fish often kept with them - DoJo Loaches. All these books said "they won't grow bigger than the tank permits" That is such a big lie both in DoJo and Goldfish. When I got them, they were about 4 inches long and skinny much like a snake with whiskers. They get along with Goldfish because both like activity and both need a lot of room and both eat a lot and poop a lot and both can drive other species a bit nuts watching their high activity. It sounds like yours has stayed mostly undersized for his/her age. One of my Dojo's has too - he's still only about 8" long and very skinny around but he has grown some and continues to grow. Weill they were originally in a 29 gallon community tank with primarily Gourami and Rainbow fish. Nobody was very happy. Really bad mix of fish, So I removed the DoJo'ss when they were 8" long to a 50 gallon tank and boy did they start to grow and I had no time to clean their tank as often as it needed (70g, 2x a week!!! Very destructive, only floating plants get left alone - everything else got pulled and pushed around. They've become a very close family unit. 3 of the 4 have grown to 12-15 long. The runt is still only 8-9 inches. The rarely bully each other except at feeding time and only just a little - they actually more often are likely to wake up each other to eat,
So about 6 months after living in the 50 gallon tank I just bought 90 gallon tank, stand and a canister filter because the hang on back filters weren't cutting it. I have a 800 watt heater and some fans in this tank just to keep it at a cooler 75-78 degrees. (the 300 and 500 watt heaters couldn't get the water up above 65-70 degrees which is actually fine for them but too cold for me LOL. Like Goldfish they tolerate colder water just fine, I bought a $300 fluval canister vacuum filter which which I hope works well for them - they eat a LOT there is never anything left over. Flakes, ground pellets and a couple of algae wafers and pealed peas. I probably paid about $1500 for the entire set up. (Acrylic tanks were impossibly hard to get - and nobody would ship agree to ship a glass tank that big and nobody local had them in stock. I waited 8 weeks with the first tank once that arrived damaged. I waited 3 weeks for the second one, About $65 for the heater/thermometer unit which I love. A few new accessories. When they aren't jumping like dolphins they are sound asleep and dead to the world. They loved to sleep on slanted rock or bury themselves in the sand (they have no scales so they bruise easily) I found some platforms meant for turtles but are low enough to be very submerged. It just finished cycling yesterday so today I will move them to their new tank. I could have used their old media and skipped cycling but I constantly had problems with white algae covering up 100% visibility and now constant algae issues (so since I wasn't sure of the source of the white water I thought I'd start from scratchl With the use of a bottle of bacterria I had it cycled in a little more than 30 days.
So. just a warning - this COULD be your future with your goldfish. Monitor how fast they outgrow their next size tank (maybe a 29 gallon- they really don't take up that much space). Then you might have to move to a 50 gallon, then a 90-100 gallon tank like I've had to go through. Some may stay small (like one of my loaches) but others might have a big growth spurt.
Post back and let us know what you end up doing. Best of luck, and start saving. fishkeeping is a very expensive hobby.