Just being picky here but goldfish are members of the CARP family in which Koi also happen to be related to. And yeah, I agree, poor goldfish.
I also agree with aquariumfishguy about growth and space. If you want to maximize growth you need to supply ample water volume along with optimal water conditions. Starting off with too little water may cause stunted or abnormal growth patterns - too little water with too many fish will do this. Besides, larger amounts of water are easier to work with than smaller amounts... water params are more stable for a longer period in more water and temp swings aren't so much of an issue. The only way to get around this with small amount of water, such as your 10gallon, is to do frequent water changes... keeping the water as fresh as possible to fool the fish that they are in larger water.
In my experience Fancy goldfish (orandas, blackmoors, fantails) bought in March, if fed and cared for correctly, will reach five inches body length by August (5 months ) at an average room temperature of 65f in a 100L tank, they are remarkably fast growing and will need space to move around. They also need a very efficient filter but not too much turbulence in the water as they prefer slow moving waters. These are BIG fish on a par with large Cichlids and its a pity that shop people don't acknowledge this as many people are totally surprised at the rate of growth of these fish when cared for properly
Yes, I have 1 black moor in a 10g and he is getting crowded. Maybe you could give them to your lfs, or build a pond, or buy a much bigger tank, like 75-100g at minimum, but you should do it sooner rather than later.