Two recent studies about the psychology of eating highlight the subconscious at work. Here is the first, by a professor at the University of Chicago:
Imagine two servings of ice cream, one featuring a five-ounce cup overfilled with seven ounces, the other a ten-ounce cup filled with only eight ounces. Objectively the under-filled serving is better, because it contains more. But a study conducted by Christopher Hsee found that unless these two servings are presented side by side, the seven-ounce serving is actually considered more valuable. Apparently, people do not base their judgment on the amount of ice cream available, which is difficult to evaluate in isolation. Instead, they rely on an easy-to-evaluate cue: whether the serving is overfilled or under-filled. Overfilling evokes positive feelings while under-filling evokes negative feelings, and these feelings dictate people’s evaluations. (from “More is Not Always Better”)
The second study shows that people think about eating or drinking more in terms of a food unit (I had a soft drink) than a portion size (it’s still a single soft drink, whether it’s in a 12-ounce soft drink or a 24-ounce bottle).
In one of their experiments, researchers at the University of Pennsylvania...
...offered a large mixing bowl of [M&Ms candy] at the front desk of the concierge of an apartment building. Below the bowl hung a sign that read “Eat Your Fill” with “please use the spoon to serve yourself” written underneath.
If presented with a small spoon, most passersby would take a single scoop, even though the sign encouraged them to take more. If given a much larger spoon, the subjects would still take a single scoop, even though that one scoop contained much more candy. The subjects were inadvertently eating twice as much candy when the larger scoop happened to be in the bowl.
“It is more than just people afraid of appearing greedy. They didn’t know they were being observed,” Geier said. “We have a culturally enforced ‘consumption norm,’ which promotes both the tendency to complete eating a unit and the idea that a single unit is the proper amount to eat.” (from “Just How Much Is a Serving of Dip?”)