aakk9999 - 12:04 am on Mar 3, 2011 (gmt 0)
The site with 1000 pages got a boost.
Would that site perhaps be more narrowly focused on the subject (or a bit wider subject) of the search query? It is difficult to imagine you could have 18,000 pages being written on the same subject area without many of them being thin.
Perhaps for the bigger site the IA and site structure plays even bigger role in order for Google to determine site "sub-areas" and decide that the whole "sub-area" is dedicated to a wider subject of the query? Interlinking between less-related parts of the website may also play in this case.