Ты так и не ответил какой версии SQL...
Попробуй так:
select t.name as TableName, ds.name as FileGroupName, SUM(u.total_pages) * 8 / 1024 as SizeMB from sys.tables as t inner join sys.partitions as p on t.object_id = p.object_id inner join sys.allocation_units as u on p.partition_id = u.container_id inner join sys.data_spaces as ds on u.data_space_id = ds.data_space_id group by t.name, ds.name order by SizeMB desc