I should probably be more specific, because Miami going small is a bit different than the Spurs going small.
Miami normally replaces Shaq with Zo or Doleac. When Miami has to match up with 2 Bigs with size, like they did with the Spurs on occasion, then they might play some combination of Shaq, Doleac or Zo together.
In Dallas, Miami did very well in the first half with Walker (6'-9") at SF and Haslem (6'-8") at PF and Shaq at the 5. A bigger lineup. Dallas adjusted their screen roll (JT and Dirk) and had better movement toward the end of the 2nd quarter and throughout the rest of the game. There were also some foul concerns with Walker and Haslem trying to guard Dirk. Therefore, Miami went smaller with Posey and seldom played Walker and Haslem together in the 2nd half...until the end. Miami is fortunate to have some fairly quick, physical Forwards. Whether that will make enough of a difference in holding Dirk down some, remains to be seen.