Companies aren't allowed to choose whatever serving size they want. Serving sizes are governed by federal regulations drawn up by the FDA, which means they have

Companies aren't allowed to choose whatever serving size they want. Serving sizes are governed by federal regulations drawn up by the FDA, which means they have the force of law. 21 CFR § 101.9(b) defines general procedures for determining serving sizes: https://www.law.cornell.edu/cfr/text/21/101.9 https://www.law.cornell.edu/cfr/text/21/101.9 21 CFR § 101.12 defines specific "reference amounts" for particular food categories: https://www.law.cornell.edu/cfr/text/21/101.12 https://www.law.cornell.edu/cfr/text/21/101.12 See, for instance, § 101.9(b)(6) which says: "A product that is packaged and sold individually that contains less than 200 percent of the applicable reference amount must be considered to be a single-serving container, and the entire content of the product must be labeled as one serving."