  We revisit the studies of the isotopic shift in the charge radii of {\it
even-even} isotopes of Sn and Pb nuclei at $N$ = 82, and 126, respectively,
within the relativistic mean-field and Relativistic-Hartree-Bogoliubov
approach. The shell model is also used to estimate isotopic shift in these
nuclei, for the first time, to the best of our knowledge. The ground state
single-particle energies ($spe$) are calculated for non-linear NL3 \& NL3$^*$
and density-dependent DD-ME2 parameter sets compared with the experimental
data, wherever available. We establish a correlation between the filling of
single-particle levels and the isotopic shift in occupation probabilities. The
obtained $spe$ from the relativistic mean-field and
Relativistic-Hartree-Bogoliubov approaches are in line with those used in the
shell model and experimental data for both the Sn and Pb isotopic chains. The
shell model calculated isotopic shift agrees with relativistic mean-field and
Relativistic-Hartree-Bogoliubov approaches that explain the experimental data
quite well.
