American food is... well, its not really uniform.
Chances are huge that if you are going to a restaurant, your food is going to be unhealthy; likely loaded with preservatives and at least partially prepared days before. Yes, the portions are huge, but most of us take at least part of it home with us and put it in the fridge, where we either have it for lunch the next day, or forget about it and rediscover it in a new form two weeks later.
Though our restaurants serve pretty similar things, homes in different regions tend to cook different things. What makes a 'meal' in a psychological sense varies. You can often guess what groups of people immigrated to what area based off of the local cuisine, which is further influenced by what crops will grow well.
Another major factor is how much money a person has. (Probably like that everywhere.) Our poor do tend to eat higher processed, quicker to prepare 'foods', simply because they're immediately cheaper and easier to prepare. Cooking from scratch can be immensely cheaper down the road, but few have time to cook like this(America is also known for spending too much time at work, where people then pick up fast food on the lunch hour). Higher income people generally eat healthier, as they have the cash to prepare things that would be labeled "10 Healthy Dishes in 15 Minutes"(and always has at least 1 or 2 ingredients that financially equate to a poor person's meal).