how can Google Reader go further back in time on a Feedburner feed than the XML shows me?

If you use Google Reader for reading a feed on Google Feedburner, you can go back and back and back and … in time, but if you simply download the feed (as XML), the file is pretty finite and short.

• Q: How do they do that?
• Q: How do I get such a longer feed XML file myself? (I am more interested in this.)