Those using the bigger bowls took 42 percent more than those who used smaller bowls, according to findings presented in Las Vegas at the annual meeting of the North American Association for the Study of Obesity.

"This is especially relevant around Thanksgiving and shows that the larger the serving bowl or platter, the more people will eat," says Brian Wansink, a professor of both marketing and nutritional science.

In another study, he and colleagues had 97 teens at a weight-loss camp pour their juice into either short, wide glasses or tall, skinny glasses. Both types held 22.3 ounces of liquid. The researchers found the teens who used the short glasses poured an average of 9.6 ounces of juice; those who used the tall glasses poured 5.5 ounces.

"We tend to eyeball what we want to eat and drink, dish it up and then we mindlessly eat while we carry on a conversation or read the newspaper. The cue that we are finished eating is that our food is gone," Wansink says.

That clean-plate mentality was illustrated in a third investigation. Half of 84 volunteers ate tomato soup from a 24-ounce bowl. The other half ate from the same size bowl, except it was designed to slowly refill itself. The participants weren't aware that it was refilling because the bowl was connected to a concealed tube that ran underneath the table to large, heated soup cauldrons in the room next door.

Those who ate out of the refilling bowls had 14.7 ounces, 268 calories. Those with the normal bowls ate 8.5 ounces, 155 calories. Those with the refilling bowl didn't report feeling more full.