Another reason for the upswing in obesity, besides our overeating after we stopped smoking, is the sucker punch the recent quantity of psychotropic drugs we ingest has delivered. Most of these drugs have weight gain as a side effect.
Along with the physical weight gain is the zombification resulting from long term use of any psychotropics, making exercise more of a chore than it would otherwise have been. It may not only be the happy meal that is making us fat but also the happy pills.