Fandom growth factors.

Wow. Merlin (the BBC series) is a really small fandom.

I'm mystified. What is it that makes a fandom huge?

Certainly the reach and popularity of the show or book. No doubt that's why the Harry Potter fandom was immense.

And I've always thought that the source material needs some holes for the fix-it fans to play in. The tight writing of Battlestar Galactica left little wiggle room for fans.

Or it could be that sense of not-quite-satisfaction. That the source is enjoyable but doesn't deliver quite enough, leaving the fans wanting more. Much more.

It could be the BNFs. Who's attracted to the show. Certain writers draw rafts of fans with them.

And then there are the reviewers. I've had conversations with LOTR book fandom writers who switched to Harry Potter because there was so much more feedback. The same happened with SGA, everyone kept saying, "Wow, the readers really review!" Writers love feedback.

Is one or more of these factors missing from Merlin? Astolat, Pru, Seperis, and other fan-drawing writers have written for it. I don't think it has a smaller viewership than SGA (but I could be wrong). Certainly it's riddled with plot holes and potential for more (though maybe it's satisfying the viewers every week anyway). I haven't really written for Merlin so I don't know about the reviewing habits of the readers. It's only been three seasons as opposed to the full six seasons of SGA, so of course there aren't as many stories. Yet SG-1 was around for ten years and it was smaller than SGA as well.

Am I missing some factors? How does a fandom groundswell happen?