Portion size effects vary: The size of food units is a bigger problem than the number

While it is well-known that larger food portions lead to increased consumption (i.e., the portion size effect), previous studies confound the effect of the size and the number of food units making up the larger portion. Moreover, empirical tests of the mechanism underlying the portions size effect are scarce. In response to these shortcomings, we present three experiments that test the impact of food unit-size and unit-number on consumption of increasingly large portion sizes, and assess whether perceptions of quantity (driven by unit size or number) mediate the portion size effect. Study 1 (n = 185), tracking actual consumption, shows that the portion size effect is determined more by unit-size than unit-number. Relative consumption ratios are higher when participants were served portions made up of enlarged food units compared to more food units. Since perceived quantity and consumption are thought to be negatively related, Study 2 (n = 193) reveals that consumers’ quantity perceptions of portions are lower for unit-size (vs. unit-number) increases. Study 3 (n = 189) considers both perceived quantity and consumption and demonstrates that perceived quantity indeed mediates the effect of food portion size on consumption. Finally, this study also shows that encouraging consumers to focus perceptually on size when portion size increases are in terms of unit-size, or focus on number when portion size increases are in terms of unit-number, supports them in increasing quantity perceptions and decreasing actual consumption. Hence, manipulating the perceptual focus of consumers helps to mitigate the portion size effect. The findings contribute to literature on the portion size effect and numerosity heuristic, and provide practical insights on food packaging so to tackle the obesity crisis.