it maximises your ball/stem clearance, thereby reducing the chance of false triggers caused by 'shanking out' on the stylus stem, and increasing the EWL;
a larger ball allows a larger stem diameter, increasing the stiffness of the stylus;
the larger ruby ball reduces the effect of the surface finish of the component being inspected.
EWL can also be affected by assembly tolerances. For this reason Renishaw styli are assembled to exacting standards in controlled conditions.