Seriously? A lot. Fat burns around 4 calories/pound/day. A 900lb person, at 80% body fat (for argument's sake) will burn almost 3000 calories a day just in keeping their fat alive. Add on energy from muscles trying to mobilise that, muscle maintenance (6 calories/pound/day), ~500 just for the brain, and at least the same again for organ maintenance, cell turnover, etc, and we have a crapload of energy being burnt. These beasts have BMRs in the thousands and yet they still manage to keep gaining weight. Make no mistake, it is hard and requires constant effort to get this fat and stay there.