The Google cache typically only stored about 100KB of your page. So if you had a heavy page with lots of content, not all of that page would be seen in the Google cache. That seems to have changed at some point since I last checked, probably years ago.
Looking at the cache page of many sites, you can see Google is clearly downloading and archiving more than the first 100KB of that page.
A WebmasterWorld thread has reports where Google is caching 400KB+ pages.
I guess space is getting cheaper.
Forum discussion at WebmasterWorld.