Sauron Greater than Morgoth...

[Benjimir]

A point of difference also is that Melkor/Morgoth is removed from Middle-Earth, cast into the void, to watch apart from the body of creation for eternity. Sauron it is implied, remains in existence after the destruction of the ring, but without form, a infinitely weak spirit drifting until the end of times. (Not utterly unlike what JK Rowling imitates in her books. I still think she fancies her books as some latter day follow-up to LOTR, but that is another story.)

In the end though, Melkor/Morgoth was weak when he was finally removed from Middle-Earth. At his peak of power, Sauron was stronger than his master's final form, and well would have rivaled him to an extent. But the basic difference in what each was, Valar vs. Maiar dictates that Sauron could not become as powerful as his masters limit.

We see a pattern of diminishing through the entire body of Tolkien's Middle-Earth, from even the Valar to the end of the war of the Ring. The Mouth of Sauron is another example of this. A part of The Kings Men of Numenor, the Black Numenorian, who is briefly described in the LOTR as aspiring to follow in the path of Sauron. He is another shadow of a master that was himself a shadow of another.

For that matter, again, you can look to the Valar being directly engaged in fighting Melkor/Morgoth, and the Maiar pitted against Sauron.

Seems as though there is effectively a rule of engagement.