This is a well-known phenomenon. If you hand people different size bowls and let them self-serve, people always fill the bowl, rather than look for portions. One of the best things I ever did for portion control was switch from "dinner plates" (1.5 feet across) to "salad plates" (like 6-8") because small portions feel more satisfying when they aren't sitting on a big empty plate. Same with bowls. It was one of those holy fuck moments when I realized that normal "cereal bowls" hold 3 meals worth of shit.