The presumption that small churches don’t grow is false. Small churches do grow. Some grow numerically. Most grow spiritually. Many grow in both ways.
It’s just that when they grow numerically, we don’t call them small churches any more. We call them medium, big or megachurches.
Where do we think all the big churches came from? Those frogs started out as tadpoles. Asking why small churches aren’t growing is kind of like asking “why aren’t there any big small churches?”