They had Michael Jordan.
How many other "small" teams have won titles?
I don't think they were that small. They always had a near 7 footer playing down low (Cartwright, Longley, Perdue, Wennington, Brian
http://www.smileycentral.com/?partner=ZSzeb001_ZCxdm565MOUS Williams). Horace Grant wasn't undersized. Rodman maybe a little, but he always played much bigger than he really was, so that didn't matter. Kukoc was big enough to play the 4, and even bigger when he played the 3.
They were not a gimmick team. They didn't try to beat teams by outquciking them. They were commited to defense, rebounding, & controlling tempo. Then on the other end, they were very efficient in their halfcourt triangle offense. They could beat you a number of ways, something the Suns couldn't do on any consistent basis.
And let's not forget those Bulls teams even if someone considered them small, played fantastic defense.....something the Nelson, D'Antoni teams have never been accused of doing.