Make sure your backup is not including the previews folder - Lightroom 3 Catalog Previews.lrdata. This consists of the thumbnails and adjusted previews you see in LR, and doesn't need to be backed up. See if your backup software can avoid it.
Alternatively, my current preference is to set LR's Prefs to create a backup on exit. This is the lrcat file, the important one with all my LR work. My backup software then targets the folder containing these backups, not the one containing my actual LR catalogue.
if you want, zip the lrcat backups - they are often a fraction of the size of the uncompressed file.