  We conduct high-accuracy calculations of isotope shift (IS) factors of the
states involving the $D_1$ and $D_2$ lines in Zn II. Together with a global fit
to the available optical IS data, we extract nuclear-model-independent, precise
differential radii for a long chain of Zn isotopes. These radii are compared
with the ones inferred from muonic X-ray measurements. Some deviations are
found, which we ascribe to the deformed nature of Zn nuclei that introduces
nuclear-model dependency into radii extractions from muonic atoms. We arrive at
the conclusion that in cases where the many-body atomic calculations of IS
factors are well-established, optical determinations of differential radii are
more reliable than those extracted from the muonic X-ray measurements, opening
the door to improved determination of nuclear radii across the nuclear chart.
