GURPS Fantasy-Tech 2: Weapons of Fantasy

[Kromm]

The penalty is for the weapon being stupid, basically. Whether the blade is huge and unwieldy because it's very long but not all that wide, or very wide but not all that long, it's still huge and unwieldy. If it's really long, it tends to overbalance any sensible grip; if it's really wide, it tends to block your vision and require some pretty wide movements to stay out of your own way; and either way, you're contending with an unhealthy amount of drag.

You get the longer version when long reach is important to you. You get the wider version when the abilities to fight people standing right next to you and to use Fast-Draw are important to you.