I'd take a wild guess that there is a reason for restricting the range of diameters of the probe, probably having to do with the size of trig errors produced, although you'd have to examine all the math in the macro to find out for sure.
When the probe is vertical, and the ball diameter gets larger, the effective angle of motion of the probe takes on a certain angularity with respect to the vertical.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)