I believe I previously linked to this study that looks at eating occasions and portion sizes and their links to overweight/obesity. After thinking about it for a while, I think the results of the study can easily be misinterpreted.
The study primarily concludes that larger portion sizes have directly contributed to weight gain in the US. So the solution is for restuarants/food manufacturers to offer smaller portion sizes, right? Not so fast in my opinion.
I view the portion size effect as indirect evidence that most people are no longer to satiety. Eating has become dysregulated and so people eat past satiety.
In other words, if a person is eating to satiety and not beyond that point, then larger portion sizes should have no real impact. If you're full, then you're full. Just because there is more food leftover doesn't change anything (except maybe some personal/social guilt).
For example, I was at a restaurant the other day, and I was given large portions. I really wasn't that hungry to begin with, so I didn't finish the meal. It really wouldn't have mattered if they doubled the portion size, once I was full I quit eating.
Instead of public policy focusing on reducing portion sizes, I feel it would be a better use of that energy to instead get people eating to hunger and satiety again.