Forty million customers are served every day by America's biggest hamburger chains--McDonald's, Burger King, and Wendy's. In fact, many fast food restaurants are becoming an extension of the family kitchen. About two-thirds of all fast food is takeout, and most of that goes home. Some chains are beginning to put mobile units in unpredictable places such as Wal-Mart, hospitals, schools, and even gas stations. So, what is it about their burgers that attracts people to them so much? Following is a comparison of nutritional value, price, size ....