A study published in January 2012 in the Journal of Consumer Research discovered that people tend to serve themselves considerably less in small bowls than large bowls. Researchers theorized this was because of the Delboeuf illusion in which two circles of the same size are placed close to each other. One is surrounded by a large ring (think a large plate), and the other is surrounded by a smaller ring (think a small plate). This illusion makes the circle (i.e., our food) surrounded by the large ring appear smaller, so we end up serving ourselves too much.
A 2006 study also found that even nutritional experts dished themselves (and ate) more ice cream when given larger dinnerware. Oddly enough, when this experiment was altered so that people were eating in isolation, the dish size had no effect.