Consider all of the nations with Christians, and all within. You have large denominations like Roman Catholics who make up the largest portion of Christendom, a sizable amount of Orthodoxy and then protestants. Protestantism itself is divided into denominations. You have classical protestants which also rule out megachurches, such as the Anglicans/Episcopalians (Anglican communion forming the third largest Christian communion) Reformed/Presbyterians and Lutherans. Maybe a little sprinkling of Moravian and Methodist in there. Then you have evangelical Christianity with Baptists, non denominationals, some pentecostals. There's low church baptists who again would mostly be anti megachurch. Imagine your small rural congregation "me and my KJV" type.

Generally to get to the megachurches you need to go down the pentecostal/charismatic side of the non denominationals. Sure there's a sizable amount, but when you put all of Christianity in perspective, you can see it's a small slice.