Americans aren't very good at recognizing reasonable portion sizes anymore.

"If people could cut down on their portion sizes, this would be the single greatest way to combat the creeping obesity epidemic," says Madelyn Fernstrom, founding director of the University of Pittsburgh Medical Center Weight Management Center. "It's such a simple concept, but it's hard to do. There's so much hidden fat in food, it's hard to know what a serving size is."

And, if you think consuming more food than you should at one meal isn't a big concern, consider that just "100 calories a day more than you need adds up to 10 pounds in one year," says Miriam Pappo, clinical nutrition manager at Montefiore Medical Center in New York City. "That's only one or two tablespoons of salad dressing," she added.

A recent study of 120 healthy adults found that when people were given the right size portions, their weight-loss efforts were much more successful. Men in the study were told to eat about 1,700 calories daily, while the women were advised to eat 1,365 calories. Both groups were also told that their diet should consist of 55 percent carbohydrates, 25 percent protein and 20 percent fat.

In addition, 30 men and 30 women were given prepackaged entrees of meat and rice and were told to add two large salads, fruit and two glasses of skim milk a day. The remaining men and women were coached on making healthy choices but were allowed to select their own portions.

In two months, the women given prepackaged portions lost 12 pounds, while those who selected their own portions lost only eight pounds. The men eating prepackaged portions lost 16 pounds versus 11 pounds for those who controlled their own portions.

"People don't want to waste food. If it's on your plate, you'll probably eat it," notes Pappo. "If you went by your appetite, you'd probably only eat half of your entree. You have to change your mindset, eat slower, and get some tools to help you with portion control, like smaller plates."

If you need any more motivation to cut back on your portion sizes, Pappo points out that if you're a 130-pound woman who eats an extra 500 calories — something that's easy to do at a restaurant — you'd need to bicycle for an hour and a half to burn off those extra calories.

All-you-can-eat buffets, super-sized meals and cavernous drinks may help keep your wallet full, but they're also helping to expand your waistline. Nutrition experts say portion control is one of the biggest factors in successfully losing weight.