This is a common issue with multiple possible sources.
1. Has anyone removed unreferenced files lately? When a content object is deleted from Windchill, it leaves behind the content file in the vaults. These should be removed regularly based on file vault growth. Personally, I 'move' the unreferenced files, confirm none of them were referenced (windchill wt.fv.tools.WContentVerify), and then remove them from disk.
2. If the vaults are cleaned up regularly, are some of your users checking in more frequently than they need to? In the distant past, we trained CAD users to check in daily, sometimes multiple times a day. Today we prefer to upload regularly and check in when the content needs to be shared with others. Setting CAD to upload on save will grow the file vaults quickly, but this is manageable in conjunction with remove unreferenced files.
3. What are the largest content file sizes and how frequently do they iterate? Sort the folders by file size or query the database for the large files and frequency of upload. I once encountered a drawing that was over 1 GB because it had multiple photos embedded in it. Five iterations of the drawing due to drawing note changes added 5 GB to the database. The company quickly changed their documentation procedure in this case and purge helped clean up the unneeded iterations.
4. What is being published? Visualization representation files (OL, PVS, etc.) are small, but the additional file types (STEP, IGES, DXF, etc.) are not. One company was seeing rapid vaults usage growth because they were publishing STEP files upon check in for every Creo part in Windchill. When questioned about the policy, they only needed a small fraction of the STEP files. They were publishing just to have them in case someone wanted them. We changed the policy to allow publishing on the fly of STEP files and upon release. Then we removed a hundred GB of STEP files from Windchill.