Many of the posters have hit the highlights.
More people means there's more chances for you to catch a glimpse of someone handsome that you focus on.
Cities have options for getting places other than a car, which means more physical activity.
Big city hustle attracts active types and discourages people who don't have ambition. That can impact people's personal choices about physical condition.
I would add one more that I haven't seen yet:
Cities have more delivery options, allowing people who don't want to get out and about stay home and have necessities delivered to them. Less healthy people are more likely to make that choice, and consistently making that choice leads to worse health outcomes. It's a vicious cycle, but it means you're less likely to run into them as you are out and about.
Will the little voice in the back of my mind screaming "This is a bad idea" please yield the floor. --Mikey
Chivalry be hanged, and so will you.