SQL Server backup Size decreasing gradually in last 30 days

Hi All,

We have a client, where SharePoint runs in SQL. Here daily SQL backup has been configured in the Production for all SQL database.

The DB size is gradually getting reduced, in the past month where it has come upto 400 MB from 900 MB.

There is no process of deletion or any other happenings in the SQL server.

Please help me in this, to sort out 

Thanks in advance for the reply :)