This is nonsense. The Economist itself says that:
More important, junk food is not itself the source of the externality—the medical costs that arise from obesity. Unlike smoking, or excessive gambling and drinking, eating junk food does not directly impair the well-being of anyone else. And because obesity is determined by lack of exercise as well as calorie intake, its ultimate relationship with health costs is more tenuous than that of, say, smoking. It is possible to eat a lot of fatty food, exercise frequently and not generate any externalities. A more direct, though controversial, approach would simply be to tax people on the basis of their weight.Double Whopper, anyone?