For N. American hockey and baseball at least, one study (to be published in Journal of Sports Sciences) has now found a distinct correlation; with proportionately the most professional male baseball and hockey players -- almost three times as many as would be expected in proportion to male population alone -- coming from places of between 50,000 and 100,000 population: large enough to have the facilities, small enough to allow less-rigidly scheduled access to those facilities, perceived safe enough to allow children to play in the yards and the streets and thus to develop on their own their natural aptitudes -- in play. In contrast, only half as many professional male baseball and hockey players as would otherwise be expected come from larger cities (of above 500,000 population): far below the expected pool of talent.
The same study also shows a link with birth date, with professional athletes much more likely have birth dates in the first half of the year. Two straightforward possibilities here: first that although children and later professional players (at the initial recruitment phases) are sorted by age, children born closer to the beginning of the year will have had almost a year's more physical development than children born toward its end; and second that children born near the beginning of the year in a four-seasons country in the northern hemisphere will have acquired crawling and walking skills by the time summer arrives. (Auzzies and Kiwis: you now have a point of comparison and potential contrast :D)