Most of it comes down to long term storage. In theory prisons are supposed to concentrate on healthy food. In reality, that's just not going to hold up under the long term use most facilities insist on. You want years of storage with reasonable taste? That pretty much equals sub-fastfood levels of nutrition for the "real" meals. Then add on what they can buy, which will always be junk food. On top of that there's the fact that prisons make a profit on those purchases and want to encourage it. Plus shitty food can be used both by prisoners and the prison for motivation. And finally consider just how good companies have become in packing a lot of calories into a very small package. An obese person could literally inhale a normal person's caloric intake in a half hour or so and for very little overall cost.  That said, most fat people will lose weight if they go to prison. But it's simply that it won't really be noticeable. Going from a diet that promotes morbid obesity to one that promotes obesity isn't exactly an impressive change. Obviously there's more than enough resources there for someone to get jacked as hell. But a lazy stress eating sack of shit who goes into prison probably isn't going to be making very many positive changes on entering it.  The cost really isn't that much, which is the point. 5000 calories of total shit is cheaper than 1500 calories of freshly cooked real food.