Development of nebular sizes with time for the α = 3 sequences with various metallicities. Right ordinate: inner (R_cd) and outer radii (R_out), with the different metallicities indicated by various (overlapping) gray shades; see legend. Left ordinate: relative thicknesses, δR = (R_out−R_cd)/R_out, for the same sequences. Metallicity decreases monotonically from the bottom curve towards the top curve.