We have configured RBS in our sharepoint 2010 environment, when we calculate the database size from sharepoint its shows 90GB however the database size in SQL server is around 25GB and RBS size is around 160GB. I do not have any clue why it differs ?
There are two reasons for what you are seeing:
1) SharePoint reports on on how much data/content it believes is in the content database (i.e. it doesn't know that the BLOBs have been remoted), which I think is a good thing. SharePoint being unaware of whether the BLOB is in the content db or not is one of many great things about RBS.
2) The "extra" space being taken up in your BLOB store is likely attributable to what I've termed "BLOB bloat" in the past. It happens when SharePoint causes more than one BLOB to be written for the same upload or update operation, almost always related to Office docs or any other file type where SharePoint attempts property promotion (...the new/updated BLOB is written out and then a 2nd BLOB is written out during property promotion and then the 1st BLOB becomes an orphan). You could test out if that is the cause by simply uploading a Word doc to a doc lib whose parent content db has a RBS provider registered on it and see if it writes more than one new BLOB.
Hope that helps.