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.

Amorphous monsters that are huge or gigantic cannot squeeze under doors, or through iron bars.

Monsters that are MZ_HUGE or larger block line-of-sight like the giant turtle does in dNetHack.

Scale M1_CONCEAL with monster size, so that larger monsters with M1_CONCEAL can hide behind objects of a sufficiently high weight or those that are oc_big.

MZ_GIGANTIC monsters cannot enter any space that has nonwalkable terrain on at least one pair of opposite sides in any direction (like a doorway, with walls on opposite sides). MZ_HUGE monsters cannot enter any space that has nonwalkable terrain on 2 (or maybe 3) pairs of opposite sides (like a corridor).

In order for a random item to drop as a death drop, it must be able to have been carried by the monster. This means that newts and such won’t get many death drops and the ones they do will be small.

Specifically, this approach doesn’t involve trying extra hard to pick an appropriately sized item, because then lightweight items such as scrolls and magic markers would become too common. It merely rolls up a random item, checks whether it would be the right weight for this monster, then decides not to drop it if it’s unrealistically heavy.

Dragons, and probably all MZ_HUGE monsters, can’t enter corridor tiles or go through open doors because they’re too big. (This would probably mean that corrmaze levels in Gehennom would need to be removed or not generate huge monsters.)