The database size is allocated size, not data size. SQL Server, by design, maintains empty space within the data and log files in order to allow for growth of your database without having to go to the disk to allocate more space. It's normal. You'll regularly see the backup size different than the storage size. You can get a quick idea about the allocated size, used size and unallocated amounts by querying sys.dm_db_file_space_usage.