[William Cloud Hicklin]

It's entirely possible, indeed likely in the context of admittedly post-LR writings like Melkor/Morgoth, that as Saruman moved down the path of 'evil' (that is, selfish coercion), he expended his native power, so that as with Morgoth and Sauron much of his power had passed out of him into the apparatus of machines and Orcs that formed, in aggregate, his "power."

It might be argued that, as a corollary to the inability of Evil to create or make that which is new, it's also not possible for evil to augment its own power; all it can do is transmute it. So as Melkor of the beginning = Morgoth + the Melkor-element in all of Arda + his slaves and soldiers and creatures, and Sauron of the beginning = (later) Sauron + the Ring + his slaves, so Curunir who got off the boat = Saruman, in his own person, + his orcs and half-orcs and devices. The "Power of Saruman" is no greater than it ever was, and the inherent power in his humanoid frame is accordingly become far less, as the greater part of his aggregate power is now external to him.

Gandalf, by contrast, has never expended any of his native power to control or rule anybody; it could be said that his power actually "grew" in that he was able to persuade the Free Peoples to join their power to him, without spending himself in coercing them or asserting 'lordship.'