  Strong gravitational lensing leads to an occurrence of multiple images, with
different magnifications, of a lensed source. Those magnifications can in turn
be modified by microlensing on smaller mass scales within the lens. Recently,
measurements of the changes in the magnification ratio of the individual images
have been proposed as a powerful tool for estimation of the size and velocity
of the emission region in the lensed source. The changes of the magnification
ratios in blazars PKS1830-211 and QSO B0218+357, if interpreted as caused by a
microlensing on individual stars, put strong constraints on those two
variables. These constraints are difficult to accommodate with the current
models of gamma-ray emission in blazars. In this paper we study if similar
changes in the magnification ratio can be caused by microlensing on
intermediate size structures in the lensing galaxy. We investigate in details
three classes of possible lenses: globular clusters (GC), open clusters (OC)
and giant molecular clouds (GMC). We apply this scenario to the case of QSO
B0218+357. Our numerical simulations show that changes in magnifi- cations with
similar time scales can be obtained for relativistically moving emission
regions with sizes up to 0.01 pc in the case of microlensing on the cores of
GCs or clumps in GMCs. From the density of such structures in spiral galaxies
we estimate however that lensing in giant molecular clouds would be more
common.
