When we mashed up the data behind these maps, we confirmed the strong correlation between obesity and antibiotic prescription rates (we got an r of 0.74, for the statistically inclined). We also found a correlation between the states' median household incomes and antibiotic prescription rates: States with below-average median incomes tend to have higher antibiotic prescription rates. This makes sense, considering that high obesity rates correlate with low income levels. (You can see the data sets for antibiotic prescription rate, obesity, and median household income level here.)
Read more at Mother Jones' The Mysterious Link Between Antibiotics and Obesity.