  The partial (up to 7 %) substitution of Cd for Zn in the Yb-based
heavy-fermion material YbFe$_2$Zn$_{20}$ is known to induce a slight ($\sim 20$
%) reduction of the Sommerfeld specific heat coefficient $\gamma$ and a huge
(up to two orders of magnitude) reduction of the $T^2$ resistivity coefficient
$A$, corresponding to a drastic and unexpected reduction of the Kadowaki-Woods
ratio $A/\gamma ^2$. Here, Yb $L_{3}$-edge X-ray absorption spectroscopy shows
that the Yb valence state is close to $3+$ for all $x$, whereas X-ray
diffraction reveals that Cd replace the Zn ions only at the $16c$ site of the
$Fd\bar{3}m$ cubic structure, leaving the $48f$ and $96g$ sites with full Zn
occupation. Ab-initio electronic structure calculations in pure and Cd-doped
materials, carried out without considering correlations, show multiple
conduction bands with only minor modifications of the band dispersions near the
Fermi level and therefore do not explain the resistivity drop introduced by Cd
substitution. We propose that the site-selective Cd substitution introduces
light conduction bands with substantial contribution of Cd($16c$) $5p$ levels
that have weak coupling to the Yb$^{3+}$ $4f$ moments. These light fermions
coexist with heavy fermions originated from other conduction bands with larger
participation of Zn($48f$ and $96g$) $4p$ levels that remain strongly coupled
with the Yb$^{3+}$ local moments.
