I've heard it all now!! Nothing to do with the supersize fries & burger being consumed along with that "diet" drink of course! It's pretty obvious that portion/packet size has increased massively in the last 20/30 years or so. eg when I was growing up, if you wanted some crisps, you had a small bag of crisps. There weren't these massive "sharing" bags you get now (which people seem more than happy to consume on their own!)
No, decreased response to sugar has nothing to do with the supersize fries and burger - although the fact that you can't get a sugar hit so easily might potentially mean you're more likely to eat more lovely sweet processed food.
You can read the abstract of the study here,
It isn't just portion size although clearly in the USA that is a big issue, hence the battles against ridiculous portions, and the (evil) food industry fighting to keep them. We have changed how foods are made, made them sweeter, created 'healthy drinks' full of artificial sweeteners that reduce peoples sugar responses and make them eat more sugary foods in the long term, added more sugar to loads of foods to make them fit our current tastes for more sweet food. All manner of stuff has changed, and it isn't just about stopping people stuffing their mouths full of food. When even the things sold as 'diet' are turning out to be bad for us, you can see how people find it hard to know how to be healthy. Particularly in a world where despite the obvious evidence that eating home cooked food from ingredients is the most healthy thing to do, the vast majority of the 'information' we are given about eating relates to purchaseable branded products ie. unhealthy processed foods.