I think it's the matter of imagination and immersion. Fighting huge monsters feels very good, but it just feels strange when e.g. ancient wyrms in ADOM, whose descriptions suggest that they are gigantic, still take the same amount of squares as a rat, and they easily fit in a corridor which is so narrow that a human could not be attacked by two people coming from the same side (actually, even two rats). It is also more interesting when some monsters have properties which are really something new rather than just stronger versions of previous monsters.
Huge monsters make no sense on traditional roguelike maps with rooms connected by 1 tile wide corridors, where they would just get stuck, but there are lots of way to solve this problem in an interesting way, either by changing the map, or by giving special powers to the monsters. Like the Genetrix Vessana plant in IVAN, which does not move at all, but still the player will probably fight it in melee, since otherwise it attacks by creating lots of smaller plants next to the player, and the player usually does not have better attack options by the time when he reaches the Genetrix Vessana.
Kind of offtopic in this thread, but actually, immobile monsters seem to be an idea which is overlooked in roguelikes. One of my favorite monsters is the rosebush from Alphaman, which does nothing except smelling nicely. The nice smell lures the player character to touch the rosebush, which is painful, as it has thorns. (A great way of turning a well known flower into a interesting monster.) Also this mighty water monster from Mission Thunderbolt, who does not like to leave pools, but when enough missiles are thrown at it, it will, and try to destroy the responsible character on the land.
