Lots of teams have won with small players in their top 6 though as we've already discussed, and not all small players are soft. Vegas had a bunch of guys in their top 9 in the 6'0/190 range, Tampa had a bunch every year that they won, Pittsburgh had a bunch, etc. Shit, when you really look at it, it's the teams who specifically target size and gritensity that when they win, they're one and done. The teams who build around skill players of whatever size who have compete level stick around and contend for a bunch of years.
Nothing more useless than a big player who doesn't take advantage of his size. Small players have been small their whole lives and have been told they arent big enough their whole lives. If they're in the league at all it's because they overcame that alleged shortcoming. But some big guys get to the big leagues just by being big. Everything else about their game is wishing and hoping on the part of the team that drafted them.