I can't find any history on averages of height for NBA players... only recent averages... which they state that the current average height in the NBA is 6'6". But you can account for it lots of ways. Back in the early days of bball... it was predominantly white, African American on average are taller. And as history shows, as we've progressed as a people, our heights keep increasing. So... one could deduct that with the influx of African Americans, and the "over-time" factor... along with more modern dietary and body conditioning aids that the average heights of players would increase.
But why would they downsize?