Kippenhahn diagrams of three models of a single 25 M_⊙ star: at 0.2 Z_⊙ with efficient semiconvection (α_sc = 100, top panel), at 0.2 Z_⊙ but with inefficient semiconvection (α_sc = 1, middle panel), and at 1.0 Z_⊙ with efficient semiconvection (α_sc = 100, bottom panel). We also mark the stellar radius (red solid line, labeled on the right-hand side) to show the influence of mixing on the evolution in the HR diagram. In the solar metallicity case, even if the mixing is efficient, the model expands to R >  1000 R_⊙ during the HG phase. The resulting helium abundance profiles are shown in Fig. B.2.