Churches are shrinking because Christianity is being marginalized in our society.
A church can grow by focusing on entertaining Christians at the expense of reverence to God But, I regard mega-churches as judas goats (an animal used to lead other animals to slaughter). The vast majority of their members come from other churches, not from the unchurched or unsaved. And, these churches teach them to be secular in their attitudes and to focus on themselves, rather than God. The result is that many of them, and certainly their children, eventually leave church, and so Christianity becomes further marginalized in society.
Many "successful" churches are ran by preachers who know marketing better than the Bible.