The pressure in the outer layers of a star can vary in a wide range.
The masses of the stars lie in the range from a quarter of a solar mass up to 20 solar masses but radii range from 0.01 to 1000 solar radii. Therefore the density and with it the pressure vary in a very wide range. A physically large star (e.g. Deneb) is usually brighter and will have narrower spectral lines than a smaller star (e.g. Vega).
So we have another component of the spectral classification, the Luminosity Class.
The luminosity class is added in Roman numerals after the temperature spectral class.
|V||Dwarfs (=Main Sequence Stars)|