This is a common misconception. It is actually water quality that stunts growth in fish, not the size of the aquarium. Since nitrates and other pollutants, which act as growth inhibitors, accumulate more rapidly and to higher concentrations in smaller aquariums, they have a greater impact on fish growth and health in small aquariums. Providing proper filtration and performing frequent partial water exchanges will maximize growth and health in your fish regardless of aquarium size.