A 'bug' in the user preferences may be responsible for the limited amounts of site specific results in Google site:domain.com searches.
For example, if your preferences are set to 'Search only for pages written in these lanuguage(s): English' then you will get tiny results even if the whole site is in English. Whereas, if you set it to Search Language : Search for pages written in any lanuguage (Recommended), then Google will supply more pages in domain specific searches.
A post on Threadwatch suggests ways to "fix your tiny site : domain.com results in Google" :
"If you’ve had tiny results when you search for site:domain.com since May-July, you might be able to change that. To
find out if you can, first make a site:domain.com search in google.com. Then do the same thing at any datacenter, for
If google.com shows tiny results like in the hundreds, and the datacenter has a huge number in the thousands…or
millions, you can change the results on Google.com. It’s not site: that’s messed up, it’s Google having trouble with
the preferences. Go to http://www.google.com/preferences….
This is why google.com shows tiny results on google.com and high results in datacenters! Because our search
preferences are only set on a Google.com cookie. Google doesn’t need to fix site: but needs to relearn what English is.
If there is anything wrong with the site: results…the number is way too big!!!"