I just go by "mind your own fucking business."
Who cares if they're fat anyways? If they want to be alone for the rest of their life, or extremely unhealthy let 'em. I'm only losing weight and trimming down because I'd like to be more healthy and active in the workplace, I've lost about 40 pounds in a 5 month period (which is probably the most I've ever lost since then) and I'm only about 32 to my goal weight.
Adults are adults and have the ability to retain their own needs in life, if they want to be a huge monstrosity of viscera then who gives a fuck? Though I was never on the obesity list, but being overweight really isn't all that fun either.
Though it'd be nice to go to a WalMart and not have to see this every time I enter...