  In the light of the recent LHC data on proton-proton and lead-lead collisions
we examine the question of the multiplicity scaling of HBT radii in
relativistic nuclei and particle interactions. Within the UrQMD transport
approach we study a large variety of system sizes at different beam energies
and extract the HBT radii. In the calculation, we find a good scaling of the
radii as a function of charged particle multiplicity, if the change in the
multiplicity is caused by a change of centrality at the same energy. However,
the scaling is only approximate when the energy, $\sqrt{s}$, is changed and
breaks down when comparing pp to AA reactions.
