https://osu.ppy.sh/help/wiki/Score/
Geki:Does missing a Geki or Katu make you lose more HP than you normally would, compared to a 300 or 100?
[...] Getting a Geki will give a sizable boost to the Life Bar.
Katu or Katsu:
[...] Getting a Katu will give a small boost to the Life Bar.