"isn't a hollow bar more likely to bend than a solid bar?"
Yes, in pure bending. BUT the anti-sway bar works on the principle of a torsion spring. In torsion applications, all (for simplification) of the stress is concentrated on the outer edge of a piece of metal. So, a solid bar not only gives you a hella more weight, the weight is doing nothing toward torsional rigidity. In other words, the hollow bar will not bend.
As far as lower amount of unsprung weight. You may ask what good does that do me? Simply put the lower amount of inertia (a direct relation with mass) you have for the suspension (shocks, springs, etc) to compensate for on every bump, the quicker it reacts, thereby keeping your tires in contact with the road with the optimum amount of normal force, which is ALWAYS a good thing.
Seth
