I have had the same goldfish for ten years, and he was already fully grown when I got him. He was very shortly in a small aquarium but has been most of the time in a big filtered aquarium.
My question is why has he not grown but seems completely healthy?
There are several reasons why a Goldfish being kept in ideal conditions does not grow.
The most common cause is parasites.
Parasites can be protozoan, internal worms or trematodes such as skin and gill flukes.
As your Comet is not displaying symptoms such as fins being held closely to the body, and I assume not twitching or rubbing itself against aquarium objects, we can safely rule that out.
That does not mean your fish isn’t carrying parasites, it just means parasites are unlikely to be the cause.
Where parasites are suspected, a Praziquantel based medication such as API General Cure will deal with all three parasite types mentioned, is filter friendly and safe for fish.
You mention that your fish was fully grown when you got him.
A fully grown Comet has a body length
of about 200mm (8 inches).
Unless your aquarium is exceptionally large, I suspect your fish has a body length of 50-75mm (2-3 inches).
The fastest growth occurs in Goldfish in the first 12 months. After that, in ideal conditions they grow approximately 25mm (1 inch) a year.
If your Comet was stunted early in its life, it is unlikely to achieve full size, but it should have had some growth in 10 years.
Frequent water changes are one of the easiest ways to get Goldfish to grow. I have seen very large Goldfish in very small aquariums because the owners made very frequent water changes.
Even in a large aquarium, toxins such as nitrates can build up over time as filters can't remove them. Goldfish release growth inhibiting hormones into the water which is why Goldfish never grow to any size in small bowls.
The last factor to consider is feeding.
Goldfish should be fed at least once a day and between 0.50 and 2% of their body weight at each feeding. 0.50% is really only enough to sustain a fish. I feed between 2-4%.
If fast growth is desired, and the fish are young, feeding should be at least at 4% of their body weight.
To weigh a Goldfish, place a container of water on a set of electronic scales, and then switch them on. They should read zero weight having automatically compensated for the container of water. Transfer your fish to the container, and take note of its weight.
The food should be high quality, and not stale, in other words, less than three months old from opening if it is dry prepared food such as flakes or pellets. Gel foods such as Repashy Super Gold for Goldfish and Koi is a high quality, protein rich food that meets most of the dietary needs of Goldfish.
When making dietary changes, whether it is quantity or type of food, make the changes gradually as both the fish and the filtration system have to adjust.