It's not cheap to eat well. Fresh produce, fish, whole-milk dairy (cheese, yogurt), eggs, raw nuts, dark chocolate, olive oil, berries, and grass-fed beef are not cheap and it's likely that prison food is not exactly obtained from Whole Foods.
Many prisoners get around this by doing thousands of pushups in their cell and lifting weights and such. But I don't think this Jared Fogle is the exercising type.
This could also be a self-defense mechanism. If you make yourself very obese, you are less of a target for sexual predators.
How bad is jail that you can sit around eating free came all day?