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The impact of 2-body scattering on the innermost density profiles of darkmatter haloes is well established. We use a suite of cosmological simulationsand idealised numerical experiments to show that 2-body scattering isexacerbated in situations where there are two species of unequal mass. This isa consequence of mass segregation and reflects a flow of kinetic energy fromthe more to less massive particles. This has important implications for theinterpretation of galaxy sizes in cosmological hydrodynamic simulations, whichnearly always model stars with less massive particles than are used for thedark matter. We compare idealised models as well as simulations from the EAGLEproject that differ only in the mass resolution of the dark matter component,but keep sub-grid physics, baryonic mass resolution and gravitational forcesoftening fixed. If the dark matter particle mass exceeds the mass of stellarparticles, then galaxy sizes--quantified by their projected half-mass radii,${\rm R_{50}}$--increase systematically with time until ${\rm R_{50}}$ exceedsa small fraction of the redshift-dependent mean inter-particle separation, $l$(${\rm R_{50}}\geq 0.05\times l$). Our conclusions should also apply tosimulations that adopt different hydrodynamic solvers, subgrid physics oradaptive softening, but in that case may need quantitative revision. Anysimulation employing a stellar-to-dark matter particle mass ratio greater thanunity will escalate spurious energy transfer from dark matter to baryons onsmall scales.

Energy equipartition between stellar and dark matter particles in cosmological simulations results in spurious growth of galaxy sizes