seoN00B - Though you say "Google cache pages", I assume you're asking about how to increase the number of pages indexed by Google for each site.
The site operator is notoriously buggy, and probably not at all accurate for sites with so many pages... but let's assume just for a moment, for the sake of discussion, that the figures you're seeing roughly describe what Google has indexed.
If all onsite factors are roughly equal, your inbound linking would determine how many pages Google indexes. The more actual PageRank your inbound links have, the more pages Google will see and index.
The age of the inbound links is also a factor. How old are these sites, and are they roughly the same age?
The very large number of pages makes me wonder both about the quality and uniqueness of your content, and the quality of your inbound links. This in turn makes me think that Google is not likely to index most of your pages or to keep them in the index.
The site: operator is not a very good barometer of anything - the number can vary quite a bit from day to day and week to week, even though you see no difference in search traffic sent to the site. Even more, suppose a site has 3 directories. If you get the numbers for site:example.com/directory-a/, site:example.com/directory-b/ and site:example.com/directory-c/ you will often see a higher total than you get for site:example.com which you assume should catch everything.
You can get a better idea of indexation (although not perfect) by submitting XML Sitemaps for each site through Webmaster Tools and getting your feedback there