Restaurant portions in the United States expanded substantially over several decades and have stayed large. The driver is the cost structure of a restaurant, in which the food on the plate is one of the smaller expenses.
Ingredients are a minority of the price
A restaurant's costs are dominated by rent, wages, utilities and equipment, all of which continue whether a plate is full or half empty.
The raw ingredients typically account for a modest share of the menu price, and for starch-heavy items that share is smaller still.
Adding a substantial amount of rice, pasta or potato to a plate therefore costs very little while visibly increasing what the customer receives.
Value is judged by volume, not by cooking
Diners assessing whether a meal was worth its price rely heavily on how much arrived, because quantity is easy to see and quality is harder to compare.
A restaurant competing with a neighbour can raise perceived value by plating more, and that move is far cheaper than raising ingredient quality.
Once one operator does it, the comparison shifts for everyone nearby, and the new size becomes the baseline rather than a differentiator.
Larger sizes improve the margin, not just the appeal
The cost of taking an order, cooking, plating, serving and washing up is largely the same regardless of how much food is on the plate.
Selling a bigger portion at a higher price spreads those fixed costs across more revenue, so the larger size is usually the more profitable one.
The same logic drives tiered drink sizes, where the price difference between sizes far exceeds the cost difference between them.
Portion size resets what looks normal
People consistently eat more when served more, without reporting greater fullness afterwards, and this holds across many settings.
Repeated exposure also shifts expectations, so a portion that would once have looked generous begins to look ordinary or even small.
Home servingware followed, with dinner plates and glasses growing over the same period, which quietly changed portion sizes outside restaurants too.
The countervailing pressures are recent and partial
Ingredient costs rising faster than menu prices have pushed some kitchens to trim portions rather than raise prices visibly.
Smaller-plate formats and shared dishes offer another route, since they let a table order by appetite rather than by a single fixed size.
Neither has reversed the baseline, because the underlying arithmetic that made large portions cheap to offer has not changed.