database_size will always be larger than the sum of reserved + unallocated space because it includes the size of log files, but reserved and unallocated_space consider only data pages.
Pages that are used by XML indexes and full-text indexes are included in index_size for both result sets. When objname is specified, the pages for the XML indexes and full-text indexes for the object are also counted in the total reserved and index_size results.
If space usage is calculated for a database or an object that has a spatial index, the space-size columns, such as database_size, reserved, and index_size, include the size of the spatial index.
When updateusage is specified, the SQL Server Database Engine scans the data pages in the database and makes any required corrections to the sys.allocation_units and sys.partitions catalog views regarding the storage space used by each table. There are some situations, for example, after an index is dropped, when the space information for the table may not be current. updateusage can take some time to run on large tables or databases. Use updateusage only when you suspect incorrect values are being returned and when the process will not have an adverse effect on other users or processes in the database. If preferred, DBCC UPDATEUSAGE can be run separately.
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When you drop or rebuild large indexes, or drop or truncate large tables, the Database Engine defers the actual page deallocations, and their associated locks, until after the transaction commits. Deferred drop operations do not release allocated space immediately. Therefore, the values returned by sp_spaceused immediately after dropping or truncating a large object may not reflect the actual disk space available. For more information about deferred allocations, see Dropping and Rebuilding Large Objects.
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