Dialog.txt would suggest this is true, but having looked at the damage calculations in the source code it does not seem like monster size is taken into account anywhere.
Let me clarify - Dialog.txt says that large monsters take MORE damage from "long" weapons (spear, quarterstaff, 2h sword, halberd) and LESS from "short" weapons (dagger, short sword) and that the inverse is true for small monsters. The code doesn't seem to support this.
However, like Hatter up there suggests larger targets are in fact easier to hit
, much like small targets are harder to hit (good example - try hitting a floating eye with a warhammer).
Size is taken into account with to-hit values, and smaller weapons have higher to-hit values than larger ones.
Therefore, perhaps dialog.txt holds true only because a larger target is more likely to take a hit from a big weapon and take more damage than being hit with a small weapon, and that a small target it more likely to be hit by a small weapon and thus "DPS" (per turn I guess) is going to be higher than missing it 20 times with a humongous hammer.
Here's the excerpt from dialog.txt, line 2523:
Ivan perform damage with jolliness:
Point increases chance of critical hit by +50%
Cut and blunt damage simultaneous
Long weakens against small enemies and improves against large, short
POINT CUT BLUNT | ACCURACY DURABILITY BLOCK | special
Spear P8 C0 B2 | A8 D1 Blo6 | long, 2hand, throw
Quarterstaff P3 C0 B2 | A6 D1 Blo6 | long, 2hand
Bastard sword P4 C6 B4 | A4 D7 Blo5 |
Short sword P6 C4 B2 | A6 D8 Blo7 | short, 1hand
Longsword P6 C5 B2 | A7 D6 Blo8 | 1hand
2-handed sword P1 C7 B7 | A4 D6 Blo6 | long, 2hand
Axe P0 C6 B5 | A2 D4 Blo4 |
Battle-axe P0 C7 B6 | A4 D6 Blo5 |
Halberd P7 C8 B8 | A4 D4 Blo5 | long, 2hand
Poleaxe P0 C8 B8 | A4 D4 Blo4 | long, 2hand
Mace P0 C0 B6 | A4 D4 Blo5 |
Dagger P6 C2 B0 | A8 D2 Blo1 | short, 1hand