  The parameters of the Bose-Einstein correlation function may obey an {\it
$M_t$-scaling}, as observed in $S + Pb$ and $Pb + Pb$ reactions at CERN SPS.
This $M_t$-scaling implies that the Bose-Einstein correlation functions view
only a small part of the big and expanding system. The full sizes of the
expanding system at the last interaction are shown to be measurable with the
help the invariant momentum distribution of the emitted particles. A vanishing
duration parameter can also be generated in the considered model-class with a
specific $M_t$ dependence.
