ABC News' Dave Zinczenko reports:
We've always been told to clean our plates, but is that advice making us fat?
Whether it's hamburgers, mac and cheese, or mashed potatoes, American staples come in many different sizes, so what exactly is one serving?
"All we need to do is say, 'Let me look at the size and not the labels,'" said Brian Wansink, a marketing professor who runs the Food and Brand Lab at Cornell University.
He held a serving size demonstration to determine what effect "knowing" your food's portion size has on how much you actually eat.
In a demonstration for ABC News, Wansink invited 12 students who believed ABC News was conducting interviews for a profile on the professor.
Wansink offered lunch to the students, giving everyone eight ounces of pasta.
Some students were told they were getting a half-sized portion, others were told they were getting a regular-sized portion, and the rest were told they were getting a double.
"If you tell them that they've got a small-size portion, they [think], 'Oh my gosh, I'm going to finish it all and I don't have to feel guilty,'" Wansink said.
And that's exactly what happened.
The groups that were told they were eating half-sized and regular-sized portions cleaned their plates, while the double-sized portion group had leftovers.
"I was thinking that I probably wouldn't want to eat all of it," said one of the students who had been told they were eating a double-sized portion. "A double portion doesn't sound like a portion I should eat."
Wansink said this can explain a large part of the nation's obesity problem: Americans eat what they're served without thinking about how much is actually on their plates, which can be a problem considering restaurants across the country sometimes serve huge portions.
So what is the right amount of food?
Experts say the protein and carbohydrate portions of the meal should be about the size of the eater's palm. There's no limit on vegetables.
"The easiest way to pick the right amount to eat is to use a small plate," Wansink said. "That way we think we're getting a full plate, and a full serving, but we're actually tricking ourselves into eating about 23 percent less than we otherwise would."
The difference between leaving a little bit of that pasta on a plate and not can add up.
The students who were tricked into cleaning their plates ate 140 calories more, equaling an extra 12 pounds over the course of the year.