Time Machine Backing Up Large Amounts of Data Each Time
On Lion you can use tmutil to compare Time Machine backups:

sudo tmutil compare                      # compare computer to latest backup
sudo tmutil listbackups                  # list available backups
sudo tmutil compare &lt;backup1&gt; &lt;backup2&gt;  # compare two backups from backup list


This should give you a good idea about where the volume is coming from.