  Enhanced microwave absorption, larger than that in the normal state, is
observed in fine grains of type-II superconductors (MgB$_2$ and K$_3$C$_{60}$)
for magnetic fields as small as a few $\%$ of the upper critical field. The
effect is predicted by the theory of vortex motion in type-II superconductors,
however its direct observation has been elusive due to skin-depth limitations;
conventional microwave absorption studies employ larger samples where the
microwave magnetic field exclusion significantly lowers the absorption. We show
that the enhancement is observable in grains smaller than the penetration
depth. A quantitative analysis on K$_3$C$_{60}$ in the framework of the
Coffey--Clem (CC) theory explains well the temperature dependence of the
microwave absorption and also allows to determine the vortex pinning force
constant.
