  Hanbury Brown--Twiss interferometry (HBT) provides crucial insights into both
the space-time structure and the momentum-space evolution of ultrarelativistic
nuclear collisions at freeze-out. In particular, the dependence of the HBT
radii on the transverse pair momentum $K_T$ and the system charged multiplicity
$dN_{\mathrm{ch}}/d\eta$ may reflect the mechanisms driving collective behavior
in small systems. This paper argues that certain features observed in the
multiplicity dependence of the HBT radii can be naturally understood if small
systems evolve hydrodynamically at high-multiplicity. This study thus
establishes a baseline for the multiplicity dependence of HBT in hydrodynamics
which may prove useful in discriminating between competing models of
collectivity in nuclear collisions.
