If you think buying junk food in small packages will help you eat less, look out —marketers know the truth.

Two new marketing studies found that some people tend to consume more calories when junk food portions and packages are smaller. For some, it's because they perceive small packages to be … get this … diet food.

For others, it's just the temptation of small sins.

Already in stores

Manufacturers are releasing more and more products in smaller packages. And in recent years, several brand-name products, from chips to cookies to candy, have been released in smaller packages promoted as having just 100 calories. In terms of sales, the tactic has proven successful, past research shows.

The strategy might seem counterintuitive, because in many past studies, people tended to consume more when given more. In a 2005 test, for example, people who were offered 12-inch sandwiches ate more than those given shorter sandwiches.

But one of the new studies, led by Rita Coelho do Vale at the Technical University of Lisbon, found people believe smaller packages help them "regulate hedonic, tempting consumption," but in fact their consumption can actually increase. Large packages, on the other hand, trigger concern about overeating.

The participants watched episodes of "Friends" and were told the study was about evaluating ads. Bags of potato chips — of differing sizes, of course — were slipped into the test.

The result: Smaller packages are more likely to fuel temptation. "Because they are considered to be innocent pleasures, [small packages] may turn out to be sneaky small sins," the researchers conclude.

The finding is detailed in the October 2008 issue the Journal of Consumer Research.

Diet food?

In the other new study, Maura L. Scott at the University of Kentucky and colleagues at Arizona State University assessed people's perceptions and eating habits of M&Ms in regular and miniature packages.

The participants were sorted into two groups: restrained and unrestrained eaters. Oddly, the restrained eaters — chronic dieters, basically — tended to consume more calories from mini-packs than unrestrained participants.

“While restrained eaters may be attracted to smaller foods in smaller packages initially, presumably because these products are thought to help consumers with their diets, our research shows that restrained eaters actually tend to consume more of these foods than they would of regular foods," the researchers write in same journal.