I don't know for sure, but I expect it's a standard thing from any spidering. Let's say you start at one page and find 10 links; your tracker will then have 10 items. If each of those pages has 10 links, then you can then have up to 100 items to download. And so on.

Over time you're going to start seeing duplicates (ie. things you've already archived/spidered), so the number will go down. But while you're still in the 'discovery' stage it's going to go up and up.