Yeah, I thought about that concept at first. But then, does that mean that a dagger should do the damage of a shortsword when in the hands of a Halfling, just because he's smaller than a Human? A rapier sized for a smaller character might very well be an arming sword, which by all rights has the stat block of a shortsword. Finesse, 1d6, etc. I'm not completely against the concept, but if you're doing 1d8 finesse with a meter of slim steel that requires martial training, half that length of blade that also requires martial training trades out the higher die for the Light property.