People consume what they perceive as a single serving, regardless of size.
Geier, Rozin and Doros's 2006 finding: people ate more from larger portions and more from larger individual pretzels, treating the unit as the correct amount regardless of its size.
In practice
- A glass holding 250ml is filled and drunk as one glass, and so is the one holding 175ml.
- A packet of any size is treated as a serving, which is why portion sizes rather than appetite explain most of the change in consumption.