It really isn't just the number of pages you have in your index.
True for a search that results in 100+ relevant pages. I don't care if there are 10,000 or 5,000,000 pages found for my search. But sometimes for a multiple keyword search one SE gives 25 pages and another 8. *Then* it is usually Google that is doing better. Maybe a non-English search is more likely to have few results.