This actually is within the realm of common sense and inductive/deductive reasoning if you know a bit about nutrition.
First I'll agree with you that most people lack self control, and that the increase in portion sizes is a significant factor contributing to the obesity epidemic we're seeing right now.
That said, as someone who's been tangentially involved in product engineering for years, literally everything from the packaging to the chemical composition is designed the re-enforce addictive behavior (which is intrinsic to human psychology). Serving sizes are also often deceptive as they don't clearly indicate the macronutrient content for an entire package (as they do in many other countries). Sure, 7g of sugar per serving seems ok for cookies, until you realize that's every 2 in a box of 20 and you've eaten the whole thing (70g).
Most preservatives and artificial sweeteners are actually fairly inane, despite scary-looking ingredients lists. What you should be worried about are the cheap forms of sugar, refined carbohydrates, hydrolyzed oils/trans fats (which are thankfully declining in popularity), and HFCS/cane-sugar-based drinks which average 32-68g of sugar per bottle.
All of these things cause a condition called insulin resistance, which leads to obesity and eventually if untreated, acquired diabetes. Insulin is the hormone that extracts glucose (energy) from carbohydrates (food), and turns it into a source of energy for your cells. [It is well established scientifically that the aforementioned "watch-list" all spike your insulin levels, and that instability in your insulin levels relative to blood glucose is bad.](http://www.ncbi.nlm.nih.gov/pubmed/2995635)
In layman's terms, these things make your body release disproportionate amounts of insulin compared to the amount of food you've ingested, and this has a few downstream effects; a large spike can make you feel intensely hungry (why people say "empty" foods make you eat more than you would otherwise), and most importantly insulin becomes less effective at transporting glucose. That's the beginning sign of Insulin Resistance Syndrome, and its correlation to obesity is well established.
Ultimately it's because of the composition of processed foods, not any dangerous chemical additive (with a few notable exceptions) that most long-term complications arise.
Looking back 30 years you may be correct, but looking back 500 to pre-industrial diets, you'll notice sugar was less prevalent in the food of most Anthropocenes, and fiber was much more common. The sugar lobby of the 1900's is largely responsible for its GRAS status and current abundance.
These days we mill our grains into cereals, removing any traces of fiber/micronutrients and inject large amounts of liquid sugar into most beverages for 'flavor,' having the compound effect of up-regulating what we perceive to be "sweet." Both of these things make our bodies release far more insulin than is needed to extract their energy, making us hungrier disproportionally to caloric intake and setting the stage for a diabetic future.
To the unconscious consumer, these forces are silent and automatic. For as little self control as people may have, I personally can't place sole blame on the individual when I know the entire chain of supply to be rigged against their nutritional best interests.