We hear a lot about Americans being fast food-addicted couch potatoes, and while that does play a role in the nation’s expanding waist lines, a new study concludes it’s not the driving force behind the obesity epidemic.  Instead all Americans – no matter what income level they have and no matter what neighborhood they live in – are getting bigger because food is cheaper than ever before, the study authors say. Simply put, we’re spending less of our total budget on food so we’re buying more.