Unit bias is the tendency for individuals to want to complete a unit of a given item or task. People want to finish whatever portion they have no matter the size, it is a perception of completion that is satisfying to people. This has been widely studied in regards to portion control and healthy eating. Individuals will eat more when portion sizes are larger because of their desire to complete the task.
A study by Geier, Rozin, and Doros showed that participants would eat more candy when served with a large spoon than when they were served with a small spoon even though in both instances they had no limits to how many spoonfuls they could have. Humans tend to see things as a single unit and don't take into consideration to size of the unit. A 12 ounce soda is seen as one unit while a 24 ounce soda is seen as one unit even though it is twice as large as the 12 ounce soda. This research shows that portion sizes should be kept small in order to encourage healthful eating.