"More than necessary" vs. "more than are necessary"

Both forms are grammatical, there being no need for the verb 'is' or 'are'.

Both structures are correct but, for the past fify years, "more than necessary" has come to be used more often than "more than is/are necessary". See Ngram below.

I guess, it is the form of 'ellipsis' which omits the subject of the following clause, so it's used as 'than is necessary' rather than 'than it is necessary'. ...than (it) is necessary.