Calorie-counters may not be helping themselves when they buy snacks such as cookies, chips and candies in smaller packages, according to a study on consumer behaviour.

Consumers buying smaller packages of "hedonic products," believing they are eating less, may eat more of the snack than if they had bought a regular-sized portion, says the study, to be released in the October issue of the Journal of Consumer Research.

"Yet, when tempting products came in large package formats, consumers deliberated most before consumption, were least likely to consume, and consumed the least," lead author Rita Coelho do Vale of the University of Lisbon said in the study.

As part of the study, 140 university students watched a television program with either small or large packages of potato chips set beside them.

Prior to the experiment, one group was made to think of their body image, while the other wasn't.

Each of these groups were split into two other groups — one that was given a bowl containing two large bags of chips, and one that was given a bowl containing nine small bags.

In both conditions, the overall quantity of chips was about the same, around 400 grams.

In the group that had been primed to think of body image, the researchers found the group given the small packages was twice as likely to open a bag of chips compared to the group given the large packages.

The group that was given the small packages also ate about twice the amount the group that was given a large package of chips.

The study suggests "that opportunities for potentially big sins may prevent consumers from committing them, but importantly also how presumably small sins may sneakily backfire."