My hypothesis is that in these areas there is greater competition for a partner of the opposite (or same) sex, so people are more aware of their physical condition.
I would think the opposite might be the case. If there are fewer females, then I would think you would be more inclined to 'work out' to attract another mate of good standing. In cities, there are thousands of young available women, and would mean that, while you would want to be fit, wouldn't mean that "If you lose this one, your gonna have to settle with less attractive girl B." You would have multiple A candidates in a city.