I noticed a few days ago while backing up my database that it is growing by about 50mb per day.
So I checked what tables it is, and it seems to be the jobs que that is growing so much.
You might also want to consider using before save hooks instead of Workflow for tasks that happen on save. I did the same thing once, trying to synch fields between modules and my DB grew to over 2GB with just people opening and saving records.