It is perfectly OK to indulge on occasion, sans the guilt, without gaining weight. The trick is to enjoy what you are eating, and to eat mindfully while avoiding overindulging and gaining weight in the process.
Just because you buy in bulk doesn't mean you have to eat in bulk. In fact, buying in bulk can actually help you reach your goals to healthy living by making healthy foods more budget-friendly and by giving you the tools needed to control portions.
The study concluded that varying the "regular" portions affected how much the subjects actually ate. Subjects ate more food when the portion was labeled "regular" than when it was labeled "double-size" despite the fact that the two sizes were actually the same size.
A great way -- perhaps the best way -- to cut calories is to trim your portion sizes, especially of foods that are high in calories. That would include many treats you would find at holiday parties and events.
Many of us don't understand what a healthy portion size is, and for good reason. A pasta portion in a restaurant is easily three cups, and many steaks are at least a pound. Getting used to normal-sized portions is not an easy task.