In this panel we compare Sb galaxies to show again that appearance does not correlate well with absolute luminosity.
The upper comparison is NGC 23(MBT0, i = -22m.86; SbI-II) with NGC 3031 (M81; MBT0, i = -20m.75; SbI-II); NGC 23is 2.2 mag brighter than M81, yet appears less developed even taking into account the degraded resolution due to its large redshift of 4658 km/s.
The second comparison is the pair of NGC 5394 / 95 with NGC 224 (M31; MBT0, i = -21m.66; SbI-II). The larger galaxy of the double is NGC 5395, which has a redshift of 3628 km/s, while its companion, NGC 5394, has v = 3441 km/s; hence they are a physical pair. NGC 5395 is of type SbII, has only a single pair of arms and a small nucleus, yet is MBT0, i = 1 mag brighter than the impressive Andromeda Nebula, which is more regular.
The final comparison is NGC 4725 [MBT0, i = - 22m.47; Sb/SBb(r)II] with NGC 3351 (MBT0, i = -20m.66; SBbII). These galaxies are of the same luminosity class, and except for the lower surface brightness arms in NGC 3551, look similar; yet NGC 4725 is 1.8 mag brighter than NGC 3351.
NGC 3031 M81
NGC 5394 / 95
NGC 224 M31