Well, it's not the sites that are big, its the accounts. A lot of people could have one large forum or have multiple sites. People could also have a lot of referrals making up the bulk of their weight. So it's hard to say how people get their weight.
ive got over 6k with 2 sites. one is a PR6 with only 300 pages and only 1000 weight one is a PR3 with 3500 pages and 5000 weight i still think a PR6 should have more weight than a PR3 even if the PR3 has 10x more pages!