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tokens: clear difference for the Dark Sage galaxies in Fig. C1 versus Fig. 10 is the upturn in the typical values of centrals at . This is not a sign that the analytic model breaks down; rather, what we are seeing here is a tendency for low- galaxies to have both low and low Σ 0 . This could potentially just be a reflection of the fact that this mass scale is at the simulation’s resolution limit; the median H i mass of Dark Sage galaxies occupying Millennium haloes of 100 particles is . Our Dark Sage galaxy sample with is therefore almost certainly not halo-mass complete. Current results from these galaxies should thus be taken with a grain of salt, but they should mature as Dark Sage is transitioned to higher-resolution simulations. TNG100 does not share the same feature. That is, satellites and centrals follow the same power-law-like relation for the full H i mass range (similar to the top panel of Fig. 11 ). Figure C1. Similar to Figs 10 and 11 (for the right- and left-hand panels, respectively) but now the classical H i radius has been replaced with the H i half-mass radius. For simplicity, we only show the GD14 prescription for the H i /H 2 breakdown for TNG100 (the other prescriptions give effectively the same results). Measurements of individual galaxies from our observed sample are overlaid in the top panels. Thin, dot–dashed lines in the top panel give the best-fitting linear relation to the simulated galaxies, assuming a slope of 0.5. Systematic differences seen between the simulations and observations are consistent with the standard H i size–mass relation results. For Dark Sage , satellites in low-mass haloes behave the same as centrals in their upturn in at low (cf. the top and bottom panels of Fig. C1 ). But for most satellites, i.e. those in haloes of , there is no strong upturn. To explain this, we need to understand how disc truncation would affect the – relation. As pointed out in Section 4.1 , a truncated model-1 or -2 profile with is indistinguishable from a non-truncated profile with . The higher the halo mass, the more truncated the satellite’s H i is, therefore the larger the typical fitted is, meaning the more common it is for , where . Also, because many of these satellites will have had more H i before infall, they are less likely to be biased towards low Σ 0 like Dark Sage centrals of the same current mass; in principle, unlike , truncation should not affect the best-fitting Σ 0 to [or, in this case, ]. Both effects mean satellites at low should typically have lower than centrals. The effect of halo mass on is otherwise the opposite to how it was for . That is, at fixed , satellites in higher halo masses have slightly higher on average. This can again be explained in terms of Σ 0 . Dark Sage satellites in higher-mass haloes tend to have slightly lower Σ 0 ; the medians for those in haloes of and are 10 and , respectively. While there is still a tendency for lower Σ 0 to also mean higher , the specific value of is less