The American style of service doesn't plate up the food or serve it to the customer in front of him.
The food is served in plates or bowls in the kitchen itself, and brought to the customer at his table.
So, the portions of food that go onto a plate are not decided by the customer, but by the chef in the kitchen.
Read more at Buzzle: http://www.buzzle.com/articles/different-types-of-table-service.html