Sauron's power

[Elenhin]

Letter #131 (my insertions inside brackets, not bold):

But to achieve this (his powerful empire in the Second Age, controlled by the One Ring) he (Sauron) had been obliged to let a great part of his own inherent power (a frequent and very significant motive in myth and fairy-story) pass into the One Ring. While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that power existed and was in 'rapport' with himself: he was not 'diminished'. Unless some other seized it and became possessed by it. If that happened, the new possessor could (if sufficiently strong and heroic by nature) challenge Sauron, become master of all that he had learned or done since the making of the One Ring, and so overthrow him and usurp his place.
...
...if the One Ring was actually unmade, annihilated, then its power would be dissolved, Sauron's own being would be diminished to vanishing point, and he would be reduced to a shadow, a mere memory of malicious will.

In other words, after making the Ring Sauron actually did have more power than he started with, and he was not diminished when he wasn't wearing the Ring.