Our yard is home to several different kinds of growing things, ranging from weeds to trees. For the purpose of this post, let me use three things by way of analogy: potted plants, bushes, and the big Sycamore tree in the front yard.
First are the numerous potted plants that my daughter is pouring time and attention to. She has a large cherry tomato plant, two pepper plants, and two small trees - a blood orange tree, and a lemon tree. All are in pots. (Actually, the tomato plant is dead; last night she replaced it with flowers.) The pepper plants, oddly enough, bore very little fruit during the summer, but as temperatures cooled off in the fall, they started producing--we've gotten a number of peppers off both of them. The lemon and the blood orange tree are both in their "childhood."
Plants in pots are very controlled. They either (a) bear fruit for a season, and then die, or (b) are nurtured along until they are hardy enough to transplant into the ground. If and when bad conditions come (storms, freezing temperatures), we can move them off the deck and into the garage (or even into the house). We can preserve them through things that would normally kill them--but the very thing that helps them live also means they can't spread wild over the yard.
Some churches can treat their small groups like "potted plants"; some movements are treated likewise by the organizations and leaders that wnt to see them grow. They are kept in very controlled environments. If they bear fruit, it's for a season, and perhaps the fruit is used to start new plants elsewhere (or to add to the "mother church"). Some groups are just for preserving fruit ("community groups" in a lot of Western churches). We watch over them, fret over them, observe each little individual branch, watch for worms and blight, prune them, pluck the fruit and the bad leaves, and so on. They get a lot of our attention, monitoring, and measuring--but the level of care required of every group means they can't "run wild in the yard."
Our backyard is also home to some runaway hedges. In the front, we've been pretty good about them. Once or twice a year we go through them with a power hedge trimmer and cut all the extra branches off. We don't let them run amok: we carve them down to nice, solid, dense little rectangles. We gather up all the branches and toss them away. But the hedges in the back--well, there's a small group of them that I didn't bother to trim one year, and they grew and grew. The next year, they were "too big for me to trim back"--so they grew and grew some more. Now they have become small trees. If I tried to trim them down, I'd probably kill them. So I just leave them. They shade the house, so they're fine where they are.
Some collections of small groups in some movements are like these hedges. Left alone, they're not going to "run rampant" all over the yard like dandelions. They're going to grow to a typical and certain size, and they're probably not going to get much bigger than that. Some movements have called these "palm trees." They count leaders, assuming each network started by a leader will be like these hedges--they will grow to a certain size (whether it be 100, 1,000, or 10,000 believers) over a certain time period. Knowing what leaders "do," they concentrate more on raising up leaders than on the specific numbers. Numbers change constantly, anyway, until these "palm trees" or "hedges" reach a certain threshold and level off. This leads to round numbers that are in the right order of magnitude and a leader-focused strategy.
Finally, our front yard is home to a towering Sycamore tree. This thing is huge, taller than our house. If you're going to trim it, ideally you bring in a locally-owned tree-trimming company--guys that are braver than me, who will scale the tree with power tools and trim the right branches. If the tree ever toppled, it would destroy the house. (Thankfully I think the odds of that are pretty slim.) It's a beautiful sight to behold: an awesome reminder of what things can grow into, given enough time.
These trees remind me of what church planting movements, too, can grow into--given enough time. Most movements in the world today are just a few years old. Some are not much more than tomato bushes or potted trees. Others are like the hedges: they're a bit taller than the house, but nowhere near a massive sycamore. A few are enormous, in the millions of members, with decades of experience.
Sycamores, it's true, don't "fill up the yard" like grass. But they do dominate a space, and for the purposes of our analogy sycamores do one thing that potted plants and hedges don't: they put out seeds, that float on the wind. Admittedly, my family and I are not especially enamored with the seeds from our front yard: these little bits of puff-and-fluff get into everything. But it is amazing to consider: thousands of sycamore trees are found within each these little puff that each sycamore puts out. Sycamores are sycamore-starters. And that's what the biggest, most established movements are.
Once these movements get to a certain size, it becomes almost impossible to measure the exact scope of the movement. How would you count the leaves on a sycamore tree? You could, but it might be more productive to count the number of sycamore trees in a forest or the average number of seeds a sycamore puts out. That would give you a stronger sense of what's coming.