[Abhishek R]

The logic is the same. Imagine an 8WD like 3 wide 4WD wheelbases put together. Because the center to center distance between each set of wheels is smaller (for two drivetrains of equal length), the 8WD turns better than the 6WD. Things like center of gravity and wheel type affect this, but that's the gist of it.

Edit: This assumes a center dropped drivetrain, see EricH's post below for more detail