Panel a: photoionisation pressure (P_ion) and direct radiation pressure (P_dir) of the HII regions in various galaxies (see legend) plotted as a function of region size. The P_ion values occupy the top half of the plot, and P_dir values the bottom half. SMC points are taken from Lopez et al. (2014), SMC N76 from McLeod et al. (in prep.), and the LMC points from McLeod et al. (2019). We note that uncertainties in the radii values are not given in the literature, and we therefore assume a generous 20%. We also plotted the median values from the PHANGS-MUSE survey as given in Barnes et al. (2021) by the grey diamond marker. The solid black lines indicate the linear best fits for P_ion and P_{dir,  SLUG}, with shaded grey areas for the 95% confidence regions. The dashed teal line and shaded area depict the linear fit when using P_{dir,  IMF}. The black dotted line indicates the r⁻² proportionality from Eq. (6) used to calculate P_dir. Panel b: correlation coefficient analysis for the P_{dir,  SLUG} − log₁₀(r) relation (see Sect. 7). The histogram shows the probability density distribution of the Pearson correlation coefficient using the bootstrapping and perturbation method from Curran (2014). The solid black line is the median correlation coefficient, C, found for this relation and the dashed lines show the 16th and 84th percentiles used to determine the uncertainty.