The first study found bigger restaurant portions result in people consuming more calories.
Another study found that when portion sizes of all foods served over a two-day period were increased, people kept eating more at each main meal. That means they didn't compensate for overeating the first day by reducing the amount they ate the second day.
"The bigger portions that restaurants are providing make consumers vulnerable to overeating, since most individuals eat all or most of what is served. The excess food in megaportions is not going home in doggie bags. It is, instead, fueling the obesity epidemic," Dr. Barbara Rolls, lead author of both studies, says in a news release.
Both studies were presented April 12 at the Experimental Biology conference in San Diego.
In the first study, the size of the baked ziti meal served at a cafeteria-style restaurant was varied between a standard portion and a larger serving, with 50 percent more food. The price for both meals was the same.
Customers were asked to rate their satisfaction and the appropriateness of the portion size. The customers' food intake was gauged by weighing each meal in the kitchen before and after the meal.
The study found that when people were served 50 percent more food, they ate nearly all of it -- an average of 172 more calories. People rated both the normal and larger potion sizes as equally appropriate.
In the second study, 32 men and women ate breakfast, lunch and dinner. They were given take-out snacks and water for between meals. This was done for two consecutive days a week for three weeks.
Each week, the same daily menus were served but the portion sizes varied. When portion sizes were larger, women ate 335 more calories per day and men ate 513 more calories per day. When the portion sizes were doubled, women ate 530 more calories per day and men ate 803 more calories per day.
The study found it didn't matter how much the men and women ate the previous day. If they were given larger portions, they ate more food. They didn't eat less on the second day to compensate for overeating the previous day.
Here's where you can learn more about the dangers of obesity.