Prisoners today are not as big and cut as they used to be. Many of them have to do calisthenics in secret. They get creative by using partnered-up training (yeah) and there are some that can grab ahold of the bars and do side planks, but these guys look more like Bruce Lee than Ronnie Coleman.
They can buy protein in prison. They can buy cans of tuna, etc., ~ it is very common for prisoners to have protein shakes. They can't do much in their hour outside. Depends on where they are staying.
Most of California's prisons do not allow any type of lifting, whether it be barbells or bed posts, or your roommate.
Diet does provide all the vitamins and minerals that they need, but falls short in the calorie department. They basically are doing what everyone "outside" of prison SHOULD be doing: EATING LESS!
Genetics are a huge determining factor on how big a guy gets, looks or how strong he is. Most of these guys have low bodyfat and that's about it. It's not hard to maintain low levels on a calorie-restricted diet, regardless of what you are eating.
As far as stress is concerned, they may not be going to work everyday, but they are ALL stressed out of their minds. They are stressed about being locked up, depressed, claustrophobic, worried about their kids, wondering what their old ladies are doing, worried about living the next day (depending on where they are) ~ all of these things cause HUGE amounts of stress.
If they can "accept" their situation, things become easier mentally, but this takes more mental fortitude than these guys know how to tap into, so they do hundreds of pushups and situps instead, all the while starving to death and not sleeping.
The young guys have more testosterone, but they are not using it for weight-lifting. They are using it for fighting amongst themselves and creating havoc.
Prison life = Prison death. It's a sad, sad place to be. Even the prison guards need continuous counseling just to cope with those bars.