John Tierney has found that, despite increased attention to nutrition, Americans are still over eating partly due to what he calls the "health halo." He and a colleague conducted an experiment where people were shown a picture of a meal (Applebee's Oriental Chicken Salad and a 20 ounce Pepsi if you're curious) and asked to estimate the calories. On average, the number of calories were overestimated. Another group of people were shown the same meal and two crackers labeled "Trans Fat Free." This meal had more calories than the original meal, but this group underestimated the calories in this meal due to the "Trans Fat Free" label.
So in effect, Americans are fooled into thinking anything that has "Fat Free" on it is healthier and therefore consume more calories! It is the classic case of "Coke Classic has too many calories; I'll have a Diet Coke. Oh yeah, and a brownie!"