On "The Early Show," Consumer Correspondent Susan Koeppen introduced Dr. James Painter, a nutrition professor at Eastern Illinois University.
He says Americans are on a "see food diet."
"You see it; you're going to eat it," he says.
Painter has been studying human eating habits for more than two decades.
When asked how much bigger the portions are today than in the 1950s, Painter said, "In the 1950s to now, it's at least double, and in some things, it's triple."
But Painter says Americans can lose weight and still eat the things they love -- if they downsize.
In his documentary, "Portion Size Me," two of his students went on a fast food diet for 30 days. They ate things like burgers and fries for breakfast, lunch and dinner, and still lost weight because they controlled their portions.
Painter told Koeppen, "It all comes down to portion, and we get fooled by portion by the things we choose to eat with. If we eat out of something big, we eat more. If we eat out of something smaller, we eat less."
So Koeppen put his theory to the test. She invited 10 people to an all-you-can-eat ice cream social.
Koeppen told the partygoers, "Take as much as you want as much as you think you will enjoy."
Then she split them into two groups.
The first group was given small cups, small spoons and a small scooper. They were offered toppings like chocolate and sprinkles, and each bowl was weighed.
Koeppen noted, "This group seemed to enjoy every bite and some even came back for seconds."
Then Koeppen brought in the next group. This time, they were given large bowls, larger spoons and a larger scooper. And again, she weighed how much ice cream they decided to take. No one in the second group came back for seconds.
Finally, Koeppen brought the groups together.
When asked if everyone ate enough ice cream, the groups responded in the affirmative.
Then Koeppen revealed the truth of the experiment. In the end, the group with the big bowls ate nearly twice as much.
Koeppen asked one attendee, "Are you surprised that your group ate almost twice as much?"
One woman answered, "If it was a bigger bowl, I think all of us probably would have taken even a little more."
One man who went back for seconds in the first group with the small bowls and spoons said he was surprised by the experiment's results.
He said, "When I went back for seconds, I was thinking like I'm eating too much."
Painter says the lesson here is simple: control how much you weigh by controlling how much you eat.
He said, "Smaller plates, bowls, glasses, cups. You end up eating less, you don't know you're eating less and you lose weight."
Koeppen added just by switching from a huge dinner plate to a smaller plate, you can save a bunch of calories. And instead of eating out of the bag, she said you should pour yourself a portion to avoid eating until you feel sick.
Another way to eat less is to keep a food log, Koeppen said.
"Write everything down. Keep a food log. I've been doing this for a week and it has really helped. You can see in black and white how much you're eating, so you don't forget. And you can also look and say,'Wow, I pigged out on Friday. I need to cut back today.'"
Check out Koeppen's food log below:
Tuesday: Large iced latte, egg white wrap, Greek salad with chicken, bowl of lemon chicken soup, hummus, iced tea, 2 slices of pizza, bottle of water
Wednesday: Medium latte, Cheerios with skim milk, berries, 2 veggie burgers with cheese, red pepper slices, carrots, handful of almonds, hummus with chips, steak with asparagus
Thursday: Medium latte, bagel with cream cheese, Greek yogurt, grilled cheese sandwich, carrots, mixed berries, pasta with mixed vegetables and chicken, glass of red wine
Friday: Medium latte, bowl of Greek yogurt with berries and granola, turkey sandwich, carrots, bottle of water, warm brie with toast, grilled chicken salad with walnuts and apples, handful of french fries, 2 glasses of wine
Saturday: Medium latte, cinnamon raisin bagel with cream cheese, pasta with chicken and vegetables, caramel chocolate piece, chips with salsa, 3 slices pepperoni pizza
Sunday: Medium skim latte, slice of banana bread, ham and cheese sandwich, bottle water, grilled chicken, steamed vegetables, mashed potatoes, mango, mixed berries, bottle water
Monday: Large skim latte, egg white wrap, Greek salad, cup of chicken soup, iced tea, 2 slices cheese pizza, 4 pieces of chocolate