As restaurant portion sizes have grown, weight-conscious diners wanting to control their intake have started taking more food home. But that choice is leading some well-intentioned people to overindulge because their mind is tricking them into letting their guard down.

A recent study published in the Journal of Experimental Social Psychology shows the sense of accomplishment people feel from not eating all of their food may cause them to eat more and exercise less afterward, especially when they have a large portion to start with.

Researchers from the University of Michigan and the University of Southern California conducted five studies involving portion size and leftovers in an attempt to determine what affect those factors had regarding eating behavior and exercise effort. They found that perceived consumption had a lot to do with actual behavior.

As portion sizes grew so did the amount of leftovers. A large amount of leftovers led diners to believe they consumed less than they actually did. As a result they were less compelled to compensate for that afterward in terms of exercise and subsequent eating.

"We know that growing portion sizes increase consumption, but grossly enlarged portions also cause consumers to face more and more food leftovers," said Aradhna Krishna of the University of Michigan. "Our research reveals that unconsumed food can exert meaningful influence on people's perceptions, affect, motivation and important health-related behavior."

Volunteers were given cookies in one study and they were all told how much of the cookie to eat. One group had a large cookie and the other had a small cookie. In the end, both groups ate the same amount of cookie by volume, but those in the large cookie category left a lot more cookie on their plate.

Both groups were then given a bag of 10 small cookies and were allowed to eat as many as they wanted. Those who started with a large cookie ate more of the 10 cookies because in their mind they could afford to eat more. After all, they had left a large portion of their first cookie on the plate.

At the same time those who started with the smaller cookie ate fewer cookies because by proportion their perceived consumption was higher. After all, they ate almost the entire cookie they were given, even though it was small.

Participants in another study were asked to exercise as long as they believed necessary to compensate for the cookie they ate. Even though both groups ate the same amount of cookie as in the previous experiment, those who left a large portion of a large cookie uneaten exercised less than those who ate almost an entire small cookie.

"It highlights that really big portions are a big problem for two reasons," said USC’s Linda Hagen. "One, people eat more from large portions to begin with, and two, our research shows that it also changes what you do after the meal. If people ate a smaller portion and finished it completely and didn't see leftovers on their plate, they might eat less to begin with and, weirdly, might feel more satiated."

Click here to read more in the Journal of Experimental Social Psychology.