How to Raise a Sports Star, from Russ Sears
Here is an article that describes a study of percentage of pro athletes from small towns.
However, from practical experience might I suggest that small town effect also comes from the proximity of practice. The fields, schools and coaches are close to the child. Think of the ease in small towns for a kid to hop on his or her bike and make it to practice or simply go to the park where other aquaintances can be found to join a game. It would seem that hover parenting gets a big boost by soccer Mom's having to attend every practice as the taxi.
And as an adult my running time was possible because of a 5 minute commute to work.