I've got to believe that the quickness is what appealed to Danny, as much as the size. They want their bigs to have the foot speed to play the defensive system, to be able to help on the perimeter and still be able to recover to their man. KG is the absolute ideal for the defensive system they run. Baby was successful because even though he's a huge guy, he's got great footwork; he's got the feet of a guard. The big men who've struggled more defensively have been guys like Powe and Big Al and Bass, who are more lumbering types.
I think they look at Melo and say that even though he'll probably always be a limited player, he's got the potential to be the team's defensive anchor (hopefully he'll get there before KG retires!). Fab's soccer background seems to be helping, too.