Why is my content database so large?

If your SharePoint site collection hasn't grown, but your content database has, the most likely culprit is versioning.  If a list -- or worse, a library -- has versioning enabled, the default is to keep every single one.  That means that every time someone edits and checks in a document, its storage footprint increases by the size of the document (and probably a little more).

The solution?  It could be a bit painful, but you'll need to go back into each library and restrict the number of versions to keep (three is sufficient for most uses, but your needs may vary).  I suggest keeping only major versions as well, since minor versions are really just stopping points on the way to a published document.

Of course if you have a real business need to keep all those versions around, then you'll want to look into an archiving solution that will take the old versions out of the content database but still make them available if necessary.