When VDBs are provisioned for the first time, they occupy almost zero storage space. They are share all the underlying blocks of the Source Snapshot.
Space usage of a VDB grows in two ways. First, data modifications (Inserts/Updates/Deletes) in the VDB take up extra storage. Changed data blocks cause the copy-on-write file-system to allocate additional blocks to store changes deviating from the original shared block.
Second, if VDBs are not Refreshed for a long period of time (longer than Source Snapshot Retention window, typically 7-30 days), then Delphix must retain blocks of the underlying Snapshot even though it might be eligible for deletion. Holding the original Source blocks for the dependent VDB translates into more Storage requirement.
To identify the date when a particular VDB was provisioned, in Filter By Keyword field on the Job History listing, enter a phrase like "provision" or "refresh". Add'l, adjust the Date Range under Settings to cover wider time period(> 1 week).
Assuming your Business allows it, refresh the older VDBs to release space. Note: VDB Refresh will delete all existing data from VDB and re-populate with latest changes from Source.
The recommended approach is to regularly refresh VDBs in a planned fashion, to reduce storage pressure. Delphix Services can assist you in predicting the space requirements for various refresh policies.