From what I have heard fish realse a hormones to stunt their own growth so they won't outgrow there enviroment... I guess if you put the fish in a new bigger tank he would stop realseing the hormone and therefore start growing again.... I am no expert though
most if not all fish will grow once they are in a larger enviroment. unless of coarse they are already fully grown, or juvenilles. Juvenilles will grow to 4-5 inches no matter what I have noticed! hope that helped!