We employ an effective kinetic description to study the space-time dynamics and development of transverse flow of small and large collision systems. By combining analytical insights in the few interactions limit with numerical simulations at higher opacity, we are able to describe the development of transverse flow from very small to very large opacities, realised in small and large collision systems. Surpisingly, we find that deviations between kinetic theory and hydrodynamics persist even in the limit of very large interaction rates, which can be attributed to the presence of the early pre-equilibrium phase. Using various modified setups of the hydrodynamic simulations, some of which motivated by insights from the Bjorken flow attractor, agreement with kinetic theory can be achieved. We discuss implications for the phenomenological description of heavy-ion collisions and the applicability of viscous hydrodynamics to describe small and large collision systems.