Ever-so-easy to eat in one sitting, a bagel can contain upwards of 500 calories. Even a more reasonably sized bagel of 140 calories is more than a serving.

"Supersize" portions may be a thing of the past, but only if you're a stickler for semantics. McDonald's eliminated the notorious category of fries and soda following the release of the movie "Super Size Me," only to introduce a similar-size Hugo. At Wendy's, what was a Biggie soda -- 32 ounces -- is now simply called "medium." Indeed, despite changes designed to "get portion-control activists off their back," says Lisa Young, Ph.D., R.D., author of "The Portion Teller Plan," portions remain as outlandish as ever. At Burger King, for instance, a "meal" consisting of a Triple Whopper with King-size fries and a Coke serves up 2,120 calories and 107 grams of fat. That's an entire day's worth of calories for an average person and about three day's worth of fat. It's no wonder two-thirds of Americans are overweight and one-third are obese.

"The foods we buy today are often two, three, even five times larger than when they were first introduced to the market," says Young. "Big portions are a 'bargain,' and portions are only getting larger," she says. Convenience foods, especially, foster the bigger-is-better culture, but research (and simple observation) shows that outsized portions prevail in every eating arena: restaurants, cafes, grocery stores, and our kitchen tables.

Of course, there's no one making us eat every last crumb -- but it's human nature to do so. "If we're presented with a plate of food, we eat it simply because it's there," says Marion Nestle, Ph.D., author of "What to Eat." Indeed, studies show that the bigger the portion, the more people eat. The upshot, research suggests, is that Americans today consume an average of 400 calories more per day than we did 30 years ago, says Yale University's Lisa Sanders, M.D., author of "The Perfect Fit Diet." If that doesn't sound like much, do the math: It adds up to an extra 146,000 calories per year.

Clearly, you can't depend on the food industry to regulate your portions, and controlling how much you eat isn't simply a matter of willpower. To take charge of your weight and health, you need to understand what constitutes a healthy portion and, just as important, determine when size matters and when you can relax. Sound impossible? It's not. Here's how.