The size of the bar is just one factor. The arm length also plays a major role in determining the actual wheel rate. A big bar with long arms isn't any stiffer than a smaller bar with short arms. Another factor is where the bar hits the LCA.

On some cars you need a huge bar just to get a little bit of a wheel rate because the bar has long arms that hit the LCA in the middle rather than the end.

If you do the design correctly you can have a lightweight system that provides plenty of roll control. Some of the factory designs are better than others, but none of them were designed to be lightweight and adjustable.