The weight part is mostly from us replacing good saturated fats with 'vegetable ' oils. After ten years of studying healthy diets I just came across some science that explains whet is happening when you eat foods high in linolic acid. It's a theory that explains how different cultures were thin even when some were high carb and others were high in fat. Soybean and other seed oils are in nearly everything now.
I could find and post some links but I'm usually late to threads and the discussion is probably about over.