A bit of thread necromancy in this post, but justifiably so as this is enworld's first search result on google for the topic of different sized reach weapons.
I'd like to say that I disagree with consensus based on how reach weapons are described in Savage Species.
Furthermore, this does not disagree with the PHB. A creature using an appropriately sized weapon doubles it's reach. However, the PHB does not explicitely talk about the reach of inapropriately sized weapons, whereas Savage Species does.
I'll admit Savage Species is a few months short of being called a 3.5 rulebook, but saying a reach weapon only doubles your reach regardless of the weapon's size is just silly. Poles and chains don't shorten because the wielder has shorter arms.
EDIT: Also, by core, Centaurs don't use any reach weapons. If they did, I'd say they would probably have a 15ft reach.