amber frederiksen , May 31, 2011; 06:06 p.m.
I believe in your Lightroom folder, under Backups, you will see all of the backups in order by date. If you use Lightroom frequently and have a large catalog over a number of years, these backups add up to take up a lot of space. I came across the same problem a year ago and realized that I needed to occasionally delete these older backups. As long as you are keeping the most recent backups, and you have only added to your catalog and not deleted folders from it that you want to keep tabs on, you should be able to delete the older backups in your Backups folder. Also, if you have more than one catalog (for example, one for personal, one for work, etc), make sure that you are keeping at least one or two copies of all catalogs. Lightroom stores these backups in the same place, but you can see the name of the catalog in the filename. I don't have a folder called "Download Backups", so I'm not sure what is in there, may be worth checking to see if there are catalog files in there.
Lastly, I would suggest that you make sure you also keep a backup of your Lightroom catalogs in a separate location from your computer and occasionally update these, too.