How did this guy get/remain so fat in the prison diet?
My guess would be he’s been buying up lots and lots of goodies from the commissary. From what I’ve read, most prisons have a canteen or other store where you can buy things like ramen noodles, snacks, etc. They’re paid for through an account funded either by your prison job wages or (more often) by friends and family who put money in it for you. Post has probably been stuffing his face with as much as he can afford to buy from the place.