Why are the wings of many strong lines, including hydrogen lines and most metal lines, stronger in Dwarfs than in Supergiants?
Collisional broadening - which includes van der Waals and Stark broadening - is more important in the higher gravity, higher pressure/density atmospheres of dwarf stars (a factor of 100-1000 higher for dwarfs vs giants of the same photospheric temperature).
These collisional effects effectively &quot;truncate&quot; radiative emission and absorption processes, leading to broadening of the frequency response of a line absorption cross-section. That means strong lines have appreciable opacity away from the line centres and means the lines sample a broader range of temperatures in the photosphere.
See https://physics.stackexchange.com/a/257471/43351