I’m guessing less power to draw the string, with longer arms and more muscles relative to the shorter one. Drawing the crossbow also takes strength, but it would mostly affect loading speed rather than the power of the shot as you can draw the string into postion a bit faster. It would depend though on what kind of crossbow you’re dealing with, so it’s not that clear cut. I’m guessing the damage decreasing is meant as an abstraction that can be applied to ranged weapons in general as more get introduced, rather than having to be adjusted for each weapon.
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The bending forward comes with having to anchor the crossbow on the ground while you draw back the string (with the manual crossbow, at least). That’s where the power comes into play, which you exert more of if you’re taller. Not sure what you mean by shorter people having better posture and balance? That’s more related to the build of your body, rather than your actual size.
