Using the term "Corn fed American" in a phrase means that the person has grown up well fed, and hyper-nourished. The people from the United States are some of the most obese, and over fed in the whole world.
People who came from other countries, which have different types of nurishment, and food types, eventually raise their children in the United States. Their children quite often grow taller and stronger than their parents because of the amount of food that they have.
A good example is native Japanese who raise children in the United States. Their children usually grow taller than their parents. A perfect example is Ms. Kawaguchi, whos parents are Japanese but she turned out to be a "corn-fed" 5' 9" tall American.