  The freeze-out conditions in the light (S+S) and heavy (Pb+Pb) colliding
systems of heavy nuclei at 160 AGeV/$c$ are analyzed within the microscopic
Quark Gluon String Model (QGSM). We found that even for the most heavy systems
particle emission takes place from the whole space-time domain available for
the system evolution, but not from the thin ''freeze-out hypersurface", adopted
in fluid dynamical models. Pions are continuously emitted from the whole volume
of the reaction and reflect the main trends of the system evolution. Nucleons
in Pb+Pb collisions initially come from the surface region. For both systems
there is a separation of the elastic and inelastic freeze-out. The mesons with
large transverse momenta, $p_t$, are predominantly produced at the early stages
of the reaction. The low $p_t$-component is populated by mesons coming mainly
from the decay of resonances. This explains naturally the decreasing source
sizes with increasing $p_t$, observed in HBT interferometry. Comparison with
S+S and Au+Au systems at 11.6 AGeV/$c$ is also presented.
