The Sage says...
Coincidentally, the Sage just replied to me along the same lines:
1. Does a large weapon with reach (for example, a glaive) retain or lose reach when it is scaled down to size: medium for small characters to use?
2. Does a medium weapon with reach (for example, a heavy lance) retain or lose reach when it is scaled down to size: small for small characters to
use?
For each size increase of a reach weapon, increase reach 5 feet. A Huge guisarme has a reach of 10 feet, for instance, while a Gargantuan guisarme has a reach of 15 feet. Decreasing the size of reach weapons quickly makes reach irrelevant. Having a reach of only 5 feet is normal for Medium-size and Small creatures. The important difference is that Tiny creatures using Small reach weapons can fight in melee as if they had 5 feet of reach, meaning they do not have to enter an opponent's area to attack.
- Ketjak