Funny that you would say that seeing as how almost all of the records down south are farm pond fish it seems.
There is no reason that a fish in captivity should not grow to be BIGGER than its wild counter parts. More, higher quality, and more frequent availability of food. Lack of predators, better h2o quality, medical attention...etc etc.
Just because some people are lax on care and husbandry does not make it the rule.
There is a big difference between a farm pond and a DIY koi pond or large fish tank. Big fish need big area. You wont see any 15-20 pound bass in tanks.... You dont even see 10 pound bass raised in captivity. Bass (and most fish) will grow to a certain size depending on the tank. If you put a bass in a 55 gallon it may reach a pound or 2 quick then slow down. You can then put that same bass in a 500 gallon and it will blow up to 5 pounds then slow down.