  Cluster production is investigated in central collisions of Ca+Ca, Ni+Ni,
96Zr+96Zr, 96Ru+96Ru, Xe+CsI and Au+Au reactions at 0.4A GeV incident energy.
We find that the multiplicity of clusters with charge Z > 2 grows quadratically
with the system's total charge and is associated with a mid-rapidity source
with increasing transverse velocity fluctuations.
