  We provide a theoretical analysis of recently measured fusion cross sections
which show a surprising enhancement at low $E_\mathrm{c.m.}$ energies for the
system $^{132}$Sn+$^{40}$Ca as compared to the more neutron-rich system
$^{132}$Sn+$^{48}$Ca. Dynamic microscopic calculations are carried out on a
three-dimensional lattice with a time-dependent density-constrained density
functional theory. There are no adjustable parameters, the only input is the
Skyrme effective NN interaction. Heavy-ion potentials $V(R)$,
coordinate-dependent mass parameters $M(R)$, and total fusion cross sections
$\sigma(E_\mathrm{c.m.})$ are calculated for both systems. We are able to
explain the measured fusion enhancement in terms of the {\it narrower width} of
the ion-ion potential for $^{132}$Sn+$^{40}$Ca, while the barrier heights and
positions are approximately the same in both systems.
