The Spurs are just as good at closing out.
They are good when they are not forced to collapse in the paint... Look I think that playing small is going to create a chain reaction that goes the following...
To help out vs James in the paint, the are going to collapse more in the paint (especially if there is no shot blocker in the game) now if they choose to keep the shot blocker in the game, then it would leave Duncan and Splitter to guard against Battier and Bosh in the perimeter, a move that may also pull the big guys out of the paint, or leaving them free from the outside..
On top of all of that, the Spurs have to worry about Wade getting into the paint and driving also.
Again not saying that it would be impossible for the Spurs to win, its just that out of the two teams the Heat can play the way they normally play, with out much of a problem, however, the Spurs have a ton of adjustments to make, and not enough flexible players to flex with..

But hes has chips. I don't get it?
