Collapsed mass inside an ionized region as a function of its volume. The gray colormap represents the distribution of halos the HIRRAH-21 simulation and its numerical mean (top panel) and dispersion (bottom panel) are shown as black lines in the corresponding panels. The inferred relations with (dashed lines) and without (solid lines) considering the sample variance are shown for both our power law (green lines) and our logarithmic Softplus (orange lines) parameterizations. The fit is done at x_H II ∼ 1% (left panel) and x_H II ∼ 3% (right panel).