Plot (from Keel) of luminosity (MB and physical size (D25 in kpc) for the galaxies in RC2. Also shown are lines that would exclude objects from being recognized as galaxies: very compact galaxies would look like stars (upper line) while galaxies with surface brightness well below the sky would also go unrecognized (LSB galaxies: lower line).
We now know luminous compact galaxies are not common; low luminosity LSB galaxies are common (e.g. dwarf spheroidals); but high luminosity LSB galaxies are not very common, though some do exist (See below).
The prototype luminous (MB ~ -22) low surface brightness (LSB) galaxy" "Malin 1". Left: normal look-up. Right: highly stretched to see low surface brightness features (images from Bothun, taken from Keel). It is now thought that these galaxies are not a major population. In this case, there is a smaller "normal" disk in the center, and the very large extended disk is similar to extended HI disks in other spirals -- except this one has associated star formation.