Here's a thought that came out of a discussion I was having with a friend last night, sparked by the realisation that neither of us felt especially part of the churches we'd been going to for the last year, whereas both of us had been core members of churches before that.
The maximum number of people who can attend a church congregation is generally limited by the size of the building, seating, etc.
The maximum number who can feel as if they belong to a church congregation is much smaller. It is generally limted by the layout of the building - specifically the number of people who can comfortably socialise in the space used, given the layout.
One big factor, for example, would be that if the seating is in tightly packed rows, only the ends are effectively useful for socialising because there isn't sufficient room to turn around otherwise, and people can't move freely into or out of the row.
Just a thought, but it intuitively seems right.