Sorry I'm late this week - I was at a convention.
There are dozens of ways to break down conventions - for purposes of this post, I'm going to define "conventions" as "conventions for and about enthusiasts." Because that's what I'm talking about when I say "convention."
Yes, GAMA GTS is technically a convention - but it's not for enthusiasts, so it doesn't qualify as such for this discussion. GAMA GTS for publishers to show off to distributors and retailers. The fact that many of these publishers and distributors are also enthusiasts is irrelevant to the purpose of the show.
There are two yardsticks I use when discussing conventions - focus and size.
Focus is - quite simply - how narrow the focus of the convention is. The North American Discworld Convention is a very tightly-focused convention. Panels about the effects of weightlessness on coordination would not really fit here. You could probably make an argument about it fitting due to Rincewind's going off the edge of the Disc at the end of The Color of Magic, but it'd be a stretch.
By contrast, GenCon is much less focused. It's about gaming, yes, but in recent years it has expanded to include more and more aspects of gaming, including life away from the table (which, yes, does exist).
NorWesCon is even less-focused on GenCon. It's stated focus is about science fiction and fantasy. Books, TV series, movies, comics, games, and lifestyle panels are all very common.
North American Discworld Con was where I was. It was a small convention - If forced to guess, I'd say well under 1,000 people were present. But everyone there had a common bond that was pretty specific - we all love the writings of Sir Terry Pratchett.
It got me wondering: Does a tight focus for a convention limit its size? Has GenCon grown, for example, because they've added a wider variety of panels? Or did it add that variety because of its growth?
Another question which occurs to me is "at what point does your broadening of focus due to growth start to become a problem?"
My wife and I used to go to NorWesCon every year. But one year, I looked through the con program, and couldn't find any panels which interested me. The lifestyle panels had crowded out the Sci-Fi and Fantasy panels to a ridiculous extent. So we stopped going. I go to NorWes to talk about authors I enjoy and books I like with like-minded people. The problem isn't the presence of these panels - the problem is these panels outnumbering (and overwhelming) the panels which are supposedly the focus of the convention.
Looking at the panels for the last year or two, it looks like it's starting to swing back to its original focus - fewer lifestyle panels, more of its focus. I suspect that my wife and I were not alone in steering away from NorWes and their numbers dropped. Or people complained.
I suspect that tightly-focused conventions that maintain their narrow focus will tend to stay small. There are millions of Discworld readers, but like any group, only a small percentage is able and/or willing to travel for a convention.
In all honesty, there are probably more Discword readers at NorWesCon than there are at the North American Discworld Con. But very few of them are tightly focused on the Disc. They'll be fans of Niven or Barnes or Pohl or Hobb or Butcher or McGuire. For them, the Discworld is like a side dish. They may go to a Discworld panel at NorWes (although it's more likely to be a "Humorous Fantasy" panel).
This year, we are planning to give OryCon a shot. It looks like a smaller and still-focused NorWesCon, and it's not too far away.
We'll let you know how it goes.