The form that Ganondorf takes whilst in the Twilight Realm (i.e. the weird green afro form) has plenty of power. Enough to blow Hyrule Castle to smithereens. There's no reason to assume he had no power whilst in the Twilight Realm, just not enough power to get back into the world of light.
Why would he then need to house his power in the weakling Zant. Ganon has nearly unlimited power he must have been unable to use it himself in the twilight realm.