I imagine the reason for this theory is because most fish would only reach their maximum size if kept for the majority of their lifespan. The big fish live a long time, 30-40 years if my memory is correct. How many people have kept a fish for 30-40 years? Not many I'm willing to bet... so I'm sure this is where the fish never reaching maximum size thing comes from.
(and I'm sure diet has something to do with it also)
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