LI (well, really Nassau and Suffolk) has 3 million residents and Westchester has 1 million. LI def has more than 3x number of D1 players than Westchester.
Even if you (correctly) assume more than 3x number of kids on LI play lax than in Westchester, LI still has way higher % of D1 players.
Sure, some kids in Westchester are great and some clubs are good and some high school teams kick [lacrosse] but, overall, LI is way bigger, deeper, better.
Your overall point is correct, but keep in mind that Westchester has a much higher percentage of city residents (Yonkers, Mt Vernon, parts of New Rochelle, Peekskill) who, through demographics and the problems lacrosse has in urban areas, don't play lacrosse. LI, by contrast, is much more suburban by percentage. The pool of kids available in Westchester is much more than 3-1 less than on LI.
1 in 4 of every HS program in all of NYS is on LI. (I believe this excludes the Catholics)