Portion size is the amount of food served or packaged as a single serving, expressed by weight, volume, or count. It is among the most powerful determinants of how much people eat: controlled studies show that larger portions increase intake with little awareness or compensation at later meals, an effect that operates across foods, ages, and settings. Commercial portion sizes have grown substantially over recent decades, a trend implicated in rising obesity, which is why regulators and health campaigns target them through measures like the UK soft drinks industry levy, which shrank bottle sizes, and portion guidance using hand and everyday-object references. On nutrition labels, the declared serving size shapes every number on the panel, and unrealistically small servings can make products look healthier than the amounts people actually eat.