Bob Sanders reminds me a lot of Jim Leonhard. Both are undersized and yet play best against the run. Sanders was particularly valuable for the Colts when healthy because of this. Bill Polian has handed his coaching staff an undersized defense through the years. Think back to the Wild Card Game, when the Jets' big offensive line pushed Indy around in the second half and pounded the ball at will. Gang Green was just bigger and more physical. Giving that defense an extra guy to play the run at safety makes a big difference. It is a numbers game. Guys struggle to win one on ones so an extra defender provides a huge help.
Consider this. The Colts gave up 5.3 yards per run in the 2006 regular season where Sanders played 4 games. When Sanders got back for the Playoffs, that number dropped to 4.1 per run. The next year, when Sanders won Defensive Player of the Year, that number was 3.8. Sanders was second on the team in tackles in that 2007 season and third in the 2006 postseason.
Sanders is not as good in coverage, but he has been pretty successful in the box as a run guy.