List of factors that affect growth in tank off the top of my head:
Concentrations of anti-growth hormone
Frequency of temperature fluctuations
Average temp
Positive stress (Immune system, blood composition stability, etc.)
Negative stress on the same
Appetite
Diet
Genetics of the individual
Dissolved gases, wastes and neutral solids
Oxygen levels
Any and all combinations of these few factors.
Gosh, I can't stop coming up with factors! And these would vary by species and locales of collection for wild specimens! The science of providing the exact conditions for maximum growth would be a struggle to perfect, so let's just remember that any fish should get to a decent size with good, general care. Expect the maximum, and if your fish stays smaller, great.