20% is a huge number for a country the size of the DR however when you figure in the Academy's the number it is a bit skewed.
While there are 6 levels of minor league baseball (Rookie, Short Season, Low A, High A, AA & AAA) here in the states, there is a whole other minor league level played out of the Academy's in the DR.
I don't know the number of players in the league however there or over 30 teams and most players are very young (16 o 22?). Only a small percentage of those players are fed into the US based minor league system. So while 20% is a very impressive number, consider there are probably 600 to 1000 high school to college age players playing in the "minors" outside of the US included in those numbers.
Edit: Julie after a google search I found that on last years opening day rosters 11% of the MLB players were from the DR.