Okay, after some PMs from invivo, I understand what's going on.
The reason the perl downloader ends up with a MUCH bigger file that the xm360_server downloader is simply because the perl downloader is stupid.
There are TONS of duplicates in the resulting file. You see, the marketplace lists, for example, Band Hero, and Band Hero II...They both have TONS of DLC...yet, all of the DLC for each is the same. The perl scraper doesn't know this, and grabs them all (and xm360 filters the duplicates). Xm360_server understands this, builds an internal map, and only spits out the unique DLC. So, rest assured, you aren't missing anything by using the server.
As far as noting DLC that you have, but wasn't matched...It's there already. In the "DLC that you have" section of the report, the second column of the CSV file is either "HAVE", or "HAVE_BUT_NOT_IN_DB".
Lastly, donation received, thanks!