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The observation of large azimuthal anisotropies $v_n$ in the particle spectraof proton-proton (pp) and proton-nucleus (pA) collisions challenges fluiddynamic interpretations of $v_n$, as it remains unclear how small collisionsystems can hydrodynamize and to what extent hydrodynamization is needed tobuild up $v_n$. Here, we study in a simple kinetic theory how the same physicsthat leads to hydrodynamization in large systems represents itself in smallsystems. We observe that one third to one half of the elliptic flow signal seenin fully hydrodynamized systems can be built up in collisions that extend overonly one mean free path $l_{\rm mfp}$ and that do not hydrodynamize. This isqualitatively in line with observing a sizeable $v_2$ in $pp$ collisions forwhich other characteristics of soft multi-particle production seemwell-described in a free-streaming picture. We further expose a significantsystem size dependence in the accuracy of hybrid approaches that match kinetictheory to viscous fluid dynamics. The implications of these findings for areliable extraction of shear viscosity are discussed.

Opacity dependence of elliptic flow in kinetic theory