Wouldn't you think that at any point, if Sauron had felt stronger than Melkor, would have turned against him as Ungoliant did? I don't think Sauron was at any point stronger than Melkor until Melkor was cast into the Void.
However you have a point. The Silmarillion tells us how Melkor lost his power by creating Orcs, trolls, Dragions and other creatures, and giving them part of himself. He also lost power that was given to Ungoliant, and lost more power in any of his defeats or unclean victories (such as when Fingolfin attacked him).
In the other side we do read how Sauron on the opposite was not loosing but gaining power instead on the first age. I think (and that's not anymore my reading, but only my guessing) that Sauropn, after the forging of the Rings, and after taking part of the power of the Smiths of Eregion, and the Spirits of the nine men become more powerful than Sauron had been in his twilight. Howevewr, even then, the most of his power was not as a standalone entity, but as a mighty general. He managed to create with Saruman Uruk-Hai, and even his mightiest creation, the one ring. I think that Sauron, coming from more humble origins, and having experienced the Wars of the first Age, was more wise and humble than the powerful Melkor, and his tactics were more sutile...
High King of the Noldor in Middle-Earth
Honorary Knight of the Pointy Shiney
"... All those moments will be lost in time, like tears in rain. Time to die"