Generally - you want to aim for a minnimum unsprung mass. The logic behind this is that when the wheel goes over the bump , everything on the wheelp/upright side of the shock/spring sees a force. Once you've gone over that bump you want the wheel to come back down and make contact with the road as soon as possible. However because you have an unsprung proportion of the mass, this has an inertia, which will keep the wheel moving upwards until the force is countered by the spring. All the time the wheel is in the air you are losing traction.
You can of course try and use stiffer springs, but then you get to the point where the wheel no longer moves up and down and you skip over the bumps, again losing contact with the road and hence traction.
What you need to find a happy medium between the softest spring settings and the forces produced by the unsprung mass. The more you can reduce unsprung mass the better. One of the easiest ways to do this is to reduce the wheel and tyre size as they are a massive component of the unsprung mass. You may have also seen some cars that have inboard brakes adjacent to the diff - this is the same logic but has the downside of introducing huge reverse cyclic loadings into your driveshafts.
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