An overwhelming number of studies shows that people seem to base how much they eat on the amount of food or drink they're given, not how full they actually are. And portion sizes, of course, have increased dramatically over the past few decades.
In 2006, the Centers for Disease Control and Prevention published a document about research into portion sizes. Experiments with mac and cheese, sandwiches, pasta entrees, potato chip bags, bags of popcorn and other food all showed that people ate more when given more.
Often, people reported feeling just as full whether they ate smaller or larger portions. In the potato chip study, people didn't adjust their intake at dinner after eating a larger bag of chips.
The New Yorker covered more studies into the psychology of choice last August. (I recall anti-ban petitioners wandering the sticky-hot New York City parks around that time, asking passersby their opinions on the ban. Shake Shack patrons seemed to be opposed.)
People tend to go for the default option, the New Yorker reported. People living in areas where the default is to be enrolled as an organ donor are mostly organ donors. People living in areas where they have to actively check that "Be an organ donor" box are mostly not organ donors. People automatically enrolled in a retirement plan tend to stay in that plan, while people who must choose a plan move around more.
People also tend to take more when they know a bigger option exists, the New Yorker reported. It's as if knowing a super size exists, even if you don't choose it, makes that large frappe seem reasonable.