Food insecurity != being underfed. Food insecurity is a measure of a person's subjective feelings about food. If an obese person reduces his consumption from 4500 cals/day to 4000 (i.e., from 9 cheeseburgers to 8) due to perceived financial strain, he is "food insecure".
For the definition of food insecurity: http://www.washingtonpost.com/wp-srv/nation/documents/usda_r...
Go read this article to see a picture of two women suffering from food insecurity: http://www.npr.org/templates/story/story.php?storyId=9259254...
Citation showing that the average poor person is seriously overweight: http://webarchives.cdlib.org/sw1vh5dg3r/http://ers.usda.gov/...