Note also that Yahoo doesn't filter duplicate content very efficiently. So the average Wordpress weblog for instance will produce more results in yahoo than in google because it will list contents several times. Wordpress has the same contents displayed in different contexts homepage, subpage, rss, monthly archive, categories...

I think Google still counts the redundant content in terms of page count tho. For example, they will say "362 results for this" and when you browse the SERPs they say "In order to show you the most relevant results, we have omitted some entries very similar to the 155 already displayed. If you like, you can repeat the search with the omitted results included."
So while they don't show all 362 they still count them. But what I definitely think might happen is that someone who has nothing but copied content may be indexed less often.

Phillip please continue this search throughout the year. It looks like Google's pages are increasing exponentially.

In the U.S., I see 487,000,000 in Yahoo! (not 600,000,000) and 228,000,000 in Google. Any idea why your Yahoo! numbers are different than mine?