Food marketing is facing increasing challenges in using portion size (e.g., “supersizing”) as a marketing tool. Marketers have used portion size to attract customers and encourage purchase, but social agencies are expressing concern that larger portion sizes encourage greater consumption, which can cause excessive consumption and obesity. This article addresses two questions that are central to this debate: (1) How much effect does portion size have on consumption? and (2) Are there limits to this effect? A meta-analytic review reveals that, for a doubling of portion size, consumption increases by 35% on average. However, the effect has limits. An extended analysis shows that the effect of portion size is curvilinear: as portions become increasingly larger, the effect diminishes. In addition, although the portion-size effect is widespread and robust across a range of individual and environmental factors, the analysis shows that it is weaker among children, women, and overweight individuals, as well as for nonsnack food items and in contexts in which more attention is given to the food being eaten.