Large Creature wielding Spiked Chain

[Legildur]

From the SRD:

Large or larger creatures using reach weapons can strike up to double their natural reach but can't strike at their natural reach or less.

Does this line only apply to weapons that only threaten 10ft for a medium-size character?

Asssume for a moment that my spiked chain wielding dwarven monk (threatens 5ft with unarmed strike and chain, and 10ft with chain) suddenly grows to large size (10ft by 10ft) thanks to the casting of an Enlarge Person spell by the party wizard.

I know that he now threatens 5ft and 10ft with his unarmed strikes, but at what ranges does the spiked chain now threaten? Does the above line from the SRD now mean he threatens 15ft and 20ft? What about closer?

I'm guessing that with a glaive or guisarme that he couldn't threaten 5ft or 10ft (except with unarmed strike), but spiked chains threaten at natural reach as well (for a medium-size character at least).

 

[Wormwood]

I'm guessing that with a glaive or guisarme that he couldn't threaten 5ft or 10ft (except with unarmed strike), but spiked chains threaten at natural reach as well (for a medium-size character at least).

That sounds logical to me (for what it's worth).