What has always stricken me as odd is that there is a relatively small African-American congregation up the street from us in one direction and a relatively small Hispanic congregation in the other. All Episcopal and all within a five minute drive of each other. Because we are all small, none of us have the resources to do as much as we would like to do in the community and all three of us replicate a lot of the same things. For example, our church has breakfast for the needy every day. The African-American congregation does lunch several times a week. The Hispanic congregation routinely has dinners. The three churches hold services together a few times a year and it's always fantastic. Every single time I wonder why we aren't one combined parish that worships together every week. Together we could be a healthy and vibrant congregation. But the minute you start talking about closing one of the churches, or asking some vestry members to give up a little bit of their authority, well all heck breaks loose. So we all struggle on in close proximity to each other.
This is a really interesting point. Here's the reason I think the situation that you describe occurs: Church, or spiritual values, as global as they tend to be, are still very personal things to most people.
And once a church becomes a mega church, it changes. It is better than a small church for some things -- global outreach, known recognition as a spreader/"recruiter" of whatever values the church espouses, the financial wherewithal to do some major good in the world, etc.
But, it loses its appeal, and effectiveness, for some people, as well. Mostly that is because it is, or will become, less personal to them. They will not find, or feel, their place in it as much or as well. Most large churches have what are called "small groups." There's a reason for that. Some people, however, would see that and say, "If you're going to do that, why be in a big church?"
They might feel like the intimacy is forced. It's not intended that way, but the point is that, sometimes, the vibe changes as a church changes. And we all know how people usually feel about change. It's difficult.
To quote one of my mom's favorite sayings, from Mother Teresa: "We all can't do great things, but we all can do small things with great love."
Basically, it's a matter of different strokes for different folks.