Straight blades, like longsword, are design for trusting. For man-sized adn smaller creatures there is a possibility that you run trough a target. That means that part of your force is wasted. Now if you are trusting into something several feet across, like a giant, you can achieve larger trauma. An effect seen in resent times in bullet wounds. A projectile can go trough a target and still have unspent energy, which did not go into inflicting damage. As mentioned before, some shorter weapons were considered to be less effective vs larger opponents since they would have trouble reaching vital organs.
Slashing weapons like the khopesh (a battle axe sword) are made to open nice big gashes or even cutting off appendages. Imagine you swing a saber and cut a 1/2 inch stick, now using the same force cut into 2 by 4. More damage occurs.
I also like to point out that unlike western weapons, less time was spend into detailing other culture's martial heritage. A typical knopesh is 20-24 inches, but both smaller and larger examples exist. We cannot be sure on which the damage was based. Same applies to many weapons, a dagger can be 30 inches.
Last but not least, lets remember that while the authors try to base things to reality and apply logic, the game is not meant to be a complete or fully accurate representation of the universe as we know it. By that I do not mean the OP question was not good, just an attempt to prevent this discussion from denigrating into physics back argument of why weapon X is better at Z then weapon Y. Thank you and have a nice day.
