I think you are correct… that a good portion of being a “light and fluid” mover is innate… if you ever read “the blind side” (read, not watched the movie!) much of the book discusses how Michael Oher despite his massive size, moves with a spring, quickness and grace usually attributed to people 1/2 his size. He was this way living on the streets with no training, organized sports play etc (ie it was innate).
However, court mechanics aside, I think everyone can improve their lightness and quickness thru proper training. Overall body weight/mass is only one part of the equation. The other is resistance and strength training… particularly in the lower body and core… so midsection (including back!!!), quads, hams, ***, calves, etc. I believe proper training would come in the form of weight training and agility drills.