A faster, set-based solution to this problem is to join sys.dm_db_partition_stats and sys.internal_tables, just like sp_spaceused does. The code below is what runs when you generate the "Disk Usage By Table" report in Management Studio.
In my database, the set-based solution returned in 60 ms, while the cursor ran for 22 seconds.
begin try
SELECT
(row_number() over(order by a3.name, a2.name))%2 as l1,
a3.name AS [schemaname],
a2.name AS [tablename],
a1.rows as row_count,
(a1.reserved + ISNULL(a4.reserved,0))* 8 AS reserved,
a1.data * 8 AS data,
(CASE WHEN (a1.used + ISNULL(a4.used,0)) > a1.data THEN (a1.used + ISNULL(a4.used,0)) - a1.data ELSE 0 END) * 8 AS index_size,
(CASE WHEN (a1.reserved + ISNULL(a4.reserved,0)) > a1.used THEN (a1.reserved + ISNULL(a4.reserved,0)) - a1.used ELSE 0 END) * 8 AS unused
FROM
(SELECT
ps.object_id,
SUM (
CASE
WHEN (ps.index_id < 2) THEN row_count
ELSE 0
END
) AS [rows],
SUM (ps.reserved_page_count) AS reserved,
SUM (
CASE
WHEN (ps.index_id < 2) THEN (ps.in_row_data_page_count + ps.lob_used_page_count + ps.row_overflow_used_page_count)
ELSE (ps.lob_used_page_count + ps.row_overflow_used_page_count)
END
) AS data,
SUM (ps.used_page_count) AS used
FROM sys.dm_db_partition_stats ps
GROUP BY ps.object_id) AS a1
LEFT OUTER JOIN
(SELECT
it.parent_id,
SUM(ps.reserved_page_count) AS reserved,
SUM(ps.used_page_count) AS used
FROM sys.dm_db_partition_stats ps
INNER JOIN sys.internal_tables it ON (it.object_id = ps.object_id)
WHERE it.internal_type IN (202,204)
GROUP BY it.parent_id) AS a4 ON (a4.parent_id = a1.object_id)
INNER JOIN sys.all_objects a2 ON ( a1.object_id = a2.object_id )
INNER JOIN sys.schemas a3 ON (a2.schema_id = a3.schema_id)
WHERE a2.type <> N'S' and a2.type <> N'IT'
ORDER BY a3.name, a2.name
end try
begin catch
select
-100 as l1
, 1 as schemaname
, ERROR_NUMBER() as tablename
, ERROR_SEVERITY() as row_count
, ERROR_STATE() as reserved
, ERROR_MESSAGE() as data
, 1 as index_size
, 1 as unused
end catch