cause of / how to fix a table with excessive reserved pages
Please examine the level of fragmentation of those indexes with below query. Index fragmentation could be the culprit.

SELECT
 DB_NAME() AS DBName
 ,OBJECT_NAME(ps.object_id) AS TableName
 ,i.name AS IndexName
 ,ips.index_type_desc
 ,ips.avg_fragmentation_in_percent
 FROM sys.dm_db_partition_stats ps
 INNER JOIN sys.indexes i
 ON ps.object_id = i.object_id
 AND ps.index_id = i.index_id
 CROSS APPLY sys.dm_db_index_physical_stats(DB_ID(), ps.object_id, ps.index_id, null, 'LIMITED') ips
 ORDER BY ps.object_id, ps.index_id


If you rebuild your indexes, you will reclaim that extra space. You will reduce the size of those indexes and the size of the overall database. Perform index maintenance with the following script:

DECLARE @TableName varchar(255)

 DECLARE TableCursor CURSOR FOR
 (
 SELECT '[' + IST.TABLE_SCHEMA + '].[' + IST.TABLE_NAME + ']' AS [TableName]
 FROM INFORMATION_SCHEMA.TABLES IST
 WHERE IST.TABLE_TYPE = 'BASE TABLE'
 )

 OPEN TableCursor
 FETCH NEXT FROM TableCursor INTO @TableName
 WHILE @@FETCH_STATUS = 0

 BEGIN
 PRINT('Rebuilding Indexes on ' + @TableName)
 Begin Try
 EXEC('ALTER INDEX ALL ON ' + @TableName + ' REBUILD with (ONLINE=ON)')
 End Try
 Begin Catch
 PRINT('Cannot do rebuild with Online=On option, taking table ' + @TableName+' down for douing rebuild')
 EXEC('ALTER INDEX ALL ON ' + @TableName + ' REBUILD')
 End Catch
 FETCH NEXT FROM TableCursor INTO @TableName
 END

 CLOSE TableCursor
 DEALLOCATE TableCursor


Create an Azure Runbook to defragment indexes regularly and avoid using more storage space.