Storage Space When Using Dictionary-Managed Tablespaces

When creating a table or index in parallel, each parallel execution server uses the values in the STORAGE clause of the CREATE statement to create temporary segments to store the rows. Therefore, a table created with a NEXT setting of 5 MB and a PARALLEL DEGREE of 12 consumes at least 60 megabytes (MB) of storage during table creation because each process starts with an extent of 5 MB. When the parallel execution coordinator combines the segments, some of the segments may be trimmed, and the resulting table may be smaller than the requested 60 MB.
Oracle9i Data Warehousing Guide Release 2 (9.2)

http://download-west.oracle.com/docs...ngpe.htm#64260

Note: The temporary segments they are talking about in the above paragraph become table extents in the final phase. So when you create table as CTAS in DMT, it is normal that some of the segments are smaller.