Filling factors as a function of nebular size. The filling factor is computed from the Strömgren balance (Eq. (9)). Black hexagons with error bars show the median and ±1σ scatter in size bins. The dashed line marks ϵ = 1, expected for a uniform-density Strömgren sphere. The systematically low values highlight the clumpy nature of the ionised gas, with [SII] tracing dense structures embedded within more extended nebulae.