Did Growth Slow Because Our Churches Got Too Large?
It is a truism that large churches grow more slowly than small churches. One theory heard in explaining the slow down in growth among our fellowship of churches is that the large churches stopped growing as fast they had grown when smaller.

To what extent was this a factor in the decline in growth rate? 

Large churches generally do grow more slowly than small churches. This is inevitable: it is not uncommon for a small church to grow by 30% in a single year. Could a large church of, say, 5,000 members, grow at the same rate for many years? Not in even the biggest cities on earth! Such growth by a large church would swallow the population of any of the world's largest cities in just 30-odd years. No large church in all of human history has averaged 30% growth for 30 years, though it is not unheard of for small churches to experience 30% or faster growth for several years running. 

What about in the ICOC?  There is little doubt that our large churches also grew more slowly than our smaller churches. However, we would expect that, year-on-year, the smallest churches would grow at about the same (faster) rate and largest churches at about the same (slower) rate. 

In fact, though, churches of all sizes in the ICOC experienced declines in growth rates in remarkably the same way from 1994 to 2002.  We divided all churches in existence from 1994 to 2002 into four size classes:
It is surprising, therefore, that the rate of growth for all four size classes decreased in sync, as shown in the graph below, indicating church size was not a significant factor in the slowdown.  In other words, not only did the large churches grow more slowly; the medium-sized and especially small churches also grew more and more slowly.  Naturally, larger churches grew more slowly than smaller churches throughout the period, but all sizes, both large and small, experienced similar rate declines.
Looked at another way, though the number of churches increased rapidly, the combination of growth in existing churches and new plantings kept the average church size almost completely constant.
The relative portion of ICOC churches in each class stayed fairly constant.  New churches were planted, adding small churches every year.  The ratio of mid-sized churches also stayed the same, as each year many grew "up" into the larger size classes. 
1000+ Members
250-999 Members
50-249 Members
Fewer than 50 Members
Perhaps, then, another factor was at work.  A related analysis looks at church age as a possible factor. 

Latest Updates on the International Churches of Christ