The true question is why he's still alived if he was sentence to death in 1985?
He was probably no that fat back then and how can you gain that much weight by eating prison food?
Do they feed them six times a day?
He was probably on 23 hour lockdown and had people funding his commissary account, which he could use to get extra food items. You can work out in a cell if you want, you can also sit there expending very few calories and living large I would imagine.