  We investigate the charge and point-proton radii of the Ca nuclei in detail
in the density functional theory framework. As the Fayans energy density
functional provides characteristic $N$-dependence, successfully describing the
parabolic behavior of the differential charge radii in $20\leq N\leq 28$, we
pose our particular focus on its physics origin, by decomposing them into the
radial and orbital contributions. The results are compared with those from the
Skyrme plus usual pairing functional, which is taken as a representative of the
functionals having normal pairing channels. We point out that, because the
enhancement of the differential charge radii in $N<20$ with the Fayans
functional, which is contradictory with the data, has the origin parallel to
the parabolic behavior in $20\leq N\leq 28$, it is significant to describe both
$N$ regions simultaneously.
