Latter-day Saint churches with 50-100 weekly attendance grow three or four times faster than churches with 200+ weekly attendance, according to a statistic I read somewhere and can't track down.

This could be easily explained by a limited number of people who would be interested in that church. Probably, after you get to 200, you've reached out to most of the people you could, so the growth rate slows down.

To check this: split a church with 200 attendees into two churches with 100 attendees each, and see if their growth rate improves.