There are a lot of misconceptions and myths about how guys get huge in prison. Let me tell you a big "secret" - the guys in prison get huge the same way the free guys get huge. There is nothing anabolic about being in prison. The main cause of this misconception is the media and the movies.
In reality there are not a lot of huge guys in most prisons. Most prisons are a place where you would not send your worst enemy (ok...you may send him there). The food sucks and even dogs don't eat it, there are faggots everywhere and there is no equipment. Not to mention the stress levels. For guys in similar facilities there is no hope - you won't get bigger.
However it's true that some guys get jacked in prison mostly from fear and need to intimidate the others. The secret is this - train and eat. In some places you are allowed to buy "normal" food and with enough money and resources you can have access to supplements and even steroids. If you are real lucky maybe even a set of decent weights will be available.
Some of those guys are juiced, some are not but most are jacked. The secret however is certainly not prison.