  Charged particles produced in Cu+Cu collisions at \sqrt{s_{NN}} = 200 and
62.4 GeV have been measured in the PHOBOS experiment at RHIC. The comparison of
the results for Cu+Cu and Au+Au for the most central collisions at the same
energy reveals that the particle density per nucleon participant pair and the
extended longitudinal scaling behavior are similar in both systems. This
implies that for the most central events in symmetric nucleus-nucleus
collisions the particle density per nucleon participant pair does not depend on
the size of the two colliding nuclei but only on the collision energy. Also the
extended longitudinal scaling seems independent of the colliding energy and
species for central collisions. In addition, there is an overall factorization
of dN/dEta shapes as a function of collision centrality between Au+Au and Cu+Cu
collisions at the same energy.
