How the fuck do you get fat in prison??
Most people get fat in prison probably 90% of them get fat. The amount of calories you burn during the day in prison is much much lower than what you burn "on the street". In prison the vast majority of your time spent in your cell and spent laying down in your bunk watching TV or reading a book. If you have a job in prison you're usually sitting down four maybe five hours a day at that job.
I assume Silvio was at a place that still have weights since he did keep a lot of muscle so you may get one or two hours of Rec per day in which you can lift weights or walk the track or run I guess as well.
Aside from that the only cardio you get are the only time you actually move around is walking to and from Chow 3 times a day to and from work once or twice per day such as if you work in the morning walk back go to Chow and back and then back to work in the afternoon and back ( to your cell, that is). In your cell if you want to move around and stuff you're only taking three or four steps to get from your bunk too a shelf or something where you would fix your food.
So the vast majority of your time is spent laying down or sitting down. The food you get serves in chow is about 90% carbohydrates. Pastas and shit. Then you have commissary items for extra calories such as ramen noodles of course and then Little Debbie snack cakes (I favored oatmeal cream pies since you got more calories for your dollar and a slightly higher protein count), then the highest calorie count food for your dollar was peanut butter which had 3500 calories per jar. And the weightlifters would usually eat refried beans for extra protein but there's also instant rice as well and some places have bread on commissary (not everywhere because bread can be used to make Hooch).
So that's why most people get fat in prison. Extreme lack of activity and calorie expenditure as well as heavy heavy starchy carbohydrate diets and plenty of sleep.
When I was in prison I maintained about 2:30 to 2:40 body weight but the body fat percentage was higher probably around 15 or 17% and that was with legit doctor prescribed HRT 200 mg testosterone once every two weeks along with a varying T4 dosage that range between 50 micrograms and 120 micrograms per day. When I got out of prison within about a month I was already 8-10% (tested with calipers by trainer at a local gym while still just running 200mg biweekly).