Hi Abhishek,
A SAP Note 872388 contains quite detailed information regarding what steps need to be executed in every component (Adapter Engine, Integration Engine, Business Process Engine) in order to setup archiving and/or deletion. It is mentioned in the referred SAP Note and should noted that even if you only tend to use messages deletion, ensure that messages archiving job is also scheduled and executed successfully. Reason for this is, messages that were subjects for administrative action (for example, manual restart, payload change), will be ignored by deletion job, and can only be cleaned up from a database by archiving job.
Coming back to your question. It may be the case that messages have been deleted from PI database tables (as a result of archiving or deletion jobs execution), but released space has not been reclaimed on database level. In this case, you may notice that amount of records in corresponding message persistence tables has been reduced, but database disk space consumption hasn't changed. If so, please consult with your database administrators and ensure that they perform tables / tablespaces reorganization on database level. After database reorganization is completed, disk space is defragmented and reclaimed, you should be able to notice reduction of disk space consumption, followed by increase of database size caused by new messages being processed and persisted (until next effective run of messages archiving/deletion job and database reorganization job).
Regards,
Vadim