Histograms showing distribution of R_PAH vs. the logarithm of the circularized radii of the H II regions derived from HST data (from Barnes et al. 2025; panel (a)), and vs the ionization parameter tracer log([SIII]/[SII]) (panel (b)). The colors denote the number of regions in each bin. The curved dashed, solid, and dotted lines in panel (a) show models defined by Eq. (5) with the disk scale height h_d = 50, 100, and 400 pc, respectively. The red dashed line in panel (b) corresponds to Eq. (4). The anti-correlation between R_PAH and the size of the H II region is much weaker than that with the ionization parameter, and can be partially explained by the contribution of the PAHs from the diffuse ISM along the line of sight.