
Good post.
Fish, like every other living thing, will benefit from variety.
There is also a HUGE environmental/genetic involvement in the growth, too. A fish that is started out with as much high quality, protien rich food it could possibly need from the day it is hatched, will grow faster than a fish that is hatched in an environment where it has to compete to get a mouthful. A fish that has never known the stress of less than perfect water conditions will not have spent life energy dealing with ammonia/nitrites/nitrates. A fish that is grown in sufficiant water volume will not have exposure to the growth inhibitors that other fish release.
So, your fish may have been pre-conditioned towards slow growth from the day of hatching - long before you got it. It also may be genetically, simply not as big as other fish. Within a single spawning, you will find many fish that, for one reason or another (perhaps the digestion system is not as efficient or the visual accuity does not allow it to compete as well for food, or the body conformation is such that it cannot swim as fast or well as others) some of the fish are
tiny and some are
huge.
Fish will keep growing their entire lives. They grow the quickest when they are young, and slow down substantially as they age. You can encourage growth by keeping the water on the warmer side (not too warm), and feeding more often with a variety of foods - protien heavy.
Some breeders are known to feed VERY heavily to produce a large fish in as short a possible time - even, it is told, to the point of having 24 hours light, and up to 10 feedings a day. This can and will grow a fish very large in a very short time. But the fish that is produced is also said to be less that capable of breeding well, and is more than normally sensitive to many common problems.
Get a different food and try offering a second small meal each day for your fish. Keep the water squeaky clean and give in time. Fish have a way of growing when you are not looking - you may just walk in one day and go "WHOA!" The fish will have grown.