select * from (
  select t.tablespace_name,t.owner, t.segment_name, t.segment_type, sum(t.bytes / 1024 / 1024) mb
  from dba_segments t
  where t.segment_type='TABLE'
  group by t.tablespace_name,t.OWNER, t.segment_name, t.segment_type
) t
order by t.mb desc;