Numbers game
Meant to mention that although WorldCon (at around 5000 attendees) is the biggest gathering of the broad books-and-stuff side of the sf world, it's actually rather small compared to some other events.
DragonCon, for example, is the same weekend as WorldCon, and tends to bring in about 20,000 people. It's focused more on media fandom: games, movies, TV, comics, etc. I don't think I can imagine a 20,000-person con.
Burning Man this year clocked in at 29,000 people. It's not a con, of course, but there are similarities, or so I gather.
And the San Diego Comic Con apparently attracts 50,000 people every year. Ten times the size of WorldCon. The population of a small city. That's just mind-boggling.