Not to bring this thread back to life (zombie), but I've been gone for awhile, and so didn't get a chance to weigh in on this.
I'd say there are 3 factors to goldfish growth:
2- Water quality
Genetics plays the largest role in size. Never forget that goldfish have breeds, just like dogs. You can't feed a Chihuahua enough to make him a great dane. Since I love grooming ranchus, the bulldogs of the goldfish breeds, I can speak the most of their genetics. They are heavily mutated through selective breeding, but every batch (1000 eggs), yields only 50 "ranchu" looking goldfish. Did they get the genes to make them grow to 8 inches, or not? You won't know until they are a year old, and were given great food and ample space.
Water quality is always a huge factor in the first 6-12 months of their lives. This is where most ranchus get stunted. Crowded in large tanks, they age quickly, undergo their adolescent colour change and before you know it, they are irreversibly stunted. Your tank is big enough, so it isn't a factor.
Food is critical early on as well. Most "info" on goldfish is to starve them. I wrote some reviews on this forums on books about goldfish and they have no idea what they are on about. You can't feed a growing animal a tiny amount of food. Groomers know that. More food means more waste and simply means that you need to maintain the water quality with the increased bioload.
So, why are your goldfish not growing? Most likely 1- they didn't get those genes and 2- they were stunted before you got to them.
Easiest way around this is to get your breed of goldfish from the source. Find a breeder and get them to sell you fish prior to their colour change (3 month olds), feed them plenty and maintain water quality. OR you can buy a well-bred, well-groomed breed in its prime, but I love watching them grow.
I got 8 beautiful young ranchus 18-19 months ago. All were from the same batch, but 2 never got very big. Hope this helps... belatedly :P
I groom ranchus. That is all.