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Biggest one I found:
"site:yahoo.com" with 22.600.000 pages indexed.
That said, my intuitive answer would be, yes, there's probably some scale. I bet it relates to the number of backward links, as well -- a site with 20,000 pages and 10 backward links looks a lot fishier than a site with 10 pages and 10 backward links.
how come that's a "nice" number too
Because it is only an estimate (the word "about" before the number), and all their estimates only go to 3 significant digits.
Any points that google would give you for the number of pages on your site would be eclipsed by the actual effect that your internal navigation can have on how your pages rank.