In a study of NBA players, wingspan was overwhelmingly greater than height. In fact, only one active NBA player, Kurt Hinrich, is known to have a wingspan shorter than his height. John Wall, the NBA"s top pick in the 2010 draft, is under 6'3" and has a wingspan near 6'10". This is more the norm than the exception in NBA circles.
The average wingspan to height ratio in the NBA is 1.063. For medical context, 1.05 and above is criteria for Marfan Syndrome, a connective tissue disorder where the limbs grow disproportionately long.
The takeaway is that anatomical attributes are individual considerations outside of size alone for sports performance. These guys are not only giant in size. They are longer and taller - a really rare and specific physical makeup.
These stats were highlighted in The Sports Gene, by David Epstein. Fascinating read.