Amount of foods from a food group typically reported in surveys as consumed on one eating occasion;
Amount of foods that provide a comparable amount of key nutrients from that food group, for example, the amount of
cheese that provides the same amount of calcium as 1 cup fluid milk;
Amount of foods recognized by most consumers (e.g., household measures) or that can be easily multiplied or divided to describe a quantity of food actually consumed (portion);
Amount traditionally used in previous food guides to describe servings.
While the amount of food people would eat is not the only factor used, it’s a major one.
Googling quickly brings up http://www.cnpp.usda.gov/Publications/NutritionInsights/insight11.pdf
Serving size is defined as follows:
Amount of foods from a food group typically reported in surveys as consumed on one eating occasion;
Amount of foods that provide a comparable amount of key nutrients from that food group, for example, the amount of cheese that provides the same amount of calcium as 1 cup fluid milk;
Amount of foods recognized by most consumers (e.g., household measures) or that can be easily multiplied or divided to describe a quantity of food actually consumed (portion);
Amount traditionally used in previous food guides to describe servings.
While the amount of food people would eat is not the only factor used, it’s a major one.