Many snack foods now come in 100-calorie servings supposedly meant to help people control portion sizes.

But a study in the Journal of Consumer Research said they may actually cause increased snacking.

European researchers said that big bags make snackers consider how much they are eating and avoid having too much, while the snack packs are seen as harmless, making people more likely to have extra food.

As part of the study, the research team made people aware of their dietary concerns by weiging and measuring them in front of a mirror and asking questions about their diet and desire to be thin.

They were then asked to evaluate ads during TV shows, but researchers were really watching how many potato chips they ate.

Chips were available to participants in large packages or small ones.

The study found that consumption was lowest when dieting concerns were activated and package size was large. People were less likely to open large packages, and participants deliberated longer before consuming from them.