When you clean up your workspaces, or delete it, the local cache is not updated (it won't reduce in size).
Only Tools > Server Manager > Cache > Clear cache, as adviced by David, will reduce the local cache amount.
If your WS is really clean (no New Locally or Locally Modified objects), you can also delete the .cache2008 folder that resides inside the .wf folder. Next time you'll access to your WS with ProE, it will download all needed content to the local cache.
This advice is valid for WF4 and Windchill 9.1 M040 , I don't know if it's applicable to 9.0
Is there a maximum size for the .wf folder? How big can this get before
Pro/E starts hanging?
I am usually pretty good about keeping clean workspaces but right now I
have over 8 active workspaces and my .wf folder is 11GB. This morning
Pro/E is running painfully slow so I'm wondering if this is the reason.
Wildfire 4.0 Datecode m140
Windchill 9.0 Datecode m030