Rondo's huge 9 inch hands really help his handling.
Then why aren't bigs the best dribblers. I have hands that big or bigger and I was not a good dribbler. My team mates felt safer with me shooting past halfcourt than dribbling.
Big guys are not only farther from the floor--making their dribbles longer, slower, more vulnerable to defense, and further offline for the same angle--but they are also more likely to play BB because they're tall than because they're good.