I haven't really researched how many people attended the WorldCon this year, but it strikes me that 626 votes is probably average, maybe above average. Rose & I attended the 1997 WorldCon in San Antonio; we just paid at the door for one day, so we weren't eligible to vote in the Hugos. I recall that daily attendance that year averaged about 2000 people, which was a little low. The 2000 count would have included people who paid in advance for the full weekend (and thus been eligible to vote in the Hugos) and people like us who just attended one day.
And of course, not everyone eligible to vote actually does vote in a given year.