even though the tires are the ones touching the ground, it is the shocks that determine the cars stability they link the tires and axles to the chassis, the wider the shock stance the more stable the car, stand up and put you feet together ankles touching... if some one pushes you do you fall over easier? the stand feet shoulder width and same test, then stand with your feet a foot wider then your shoulders.. way more stable
now you need to think about the shock angles and how the angle helps control roll also
But doesn't a wider stance increase the leverage the wheel at it's mounting point has over the spring/shock?