DROP TABLE IF EXISTS #temp

SELECT database_id
     , avg_fragmentation_in_percent
     , page_count
     , dbsize = SUM(page_count) OVER (PARTITION BY database_id)
INTO #temp
FROM sys.dm_db_index_physical_stats(NULL, NULL, NULL, NULL, NULL)
WHERE alloc_unit_type_desc = 'IN_ROW_DATA'
    AND index_level = 0
    AND page_count > 0

SELECT db = DB_NAME(t.database_id)
     , perc = AVG(t.avg_fragmentation_in_percent)
     , data_size = MAX(t.dbsize) * 8. / 1024
FROM (
    SELECT *, rn = ROW_NUMBER() OVER (PARTITION BY database_id ORDER BY page_count * 100. / dbsize DESC)
    FROM #temp
) t
WHERE t.rn < 20
GROUP BY t.database_id