Suspect it's a bit more complex than (total calories in ) minus (total calories out). Glycaemic index and glycaemic load are probably quite important to weight, this has certainly been shown in rats and most likely works the same way in humans too. More work needed of course. One suspects that the increase in fatties and nasty lipids in the population has a lot to do with the makeup of the muck we now eat, and not neccessarily the calorific intake. Exercise is very important as it probably makes your insulin response to these unhealthy carbs a little better. And it's good for you too.
So for Farrell, off the top of my head, diet....
Prepare your own food.
Avoid saturated fats.
No added salt.
Reduce (do not eliminate) potato/rice/pasta/bread.
Oily fish couple or more times a week.
Maximise green veg, fill your plate and stomach with these.
Nice glass of red wine with a slice of cheese before you go to bed. Don't worry about the saturated fats in the cheese, one has to have some vices.
Watch weight fall off.