Giantslayer gets its bonuses against any monster that is larger than you, rather than just giants specifically. (Possibly it still gets bonuses against giants specifically even if you are as big as one.)

Note that dNetHack has a similar feature in which Giantslayer gets bonuses against any monster that’s Huge or Gigantic. It’s also been suggested to drop that to any monster of size Large or larger.

Giantslayer kills all insects adjacent to the player for a possible “Seven at one blow” effect.