thiru@9.2.0:SQL>create table t (x int) storage(initial 5m next 5m);

Table created.

-- User_Segments shows 5M, the allocated size, but there is no data yet

thiru@9.2.0:SQL>select segment_name,bytes from user_Segments where segment_name='T';

SEGMENT_NAME                        BYTES
------------------------------ ----------
T                                 5242880

thiru@9.2.0:SQL>select count(*) from t;

  COUNT(*)
----------
         0

thiru@9.2.0:SQL>analyze table t compute statistics;

Table analyzed.

-- User_tables shows 0 blocks, the maximum used 

thiru@9.2.0:SQL>select table_name,blocks from user_Tables where table_name='T';

TABLE_NAME                         BLOCKS
------------------------------ ----------
T                                       0

-- Lets insert some data

thiru@9.2.0:SQL>insert into t select rownum from all_objects;

29798 rows created.

thiru@9.2.0:SQL>commit;

Commit complete.

-- the overall size of the table doesnt increase becos it uses space within the allocated size 5M

thiru@9.2.0:SQL>select segment_name,bytes from user_Segments where segment_name='T';

SEGMENT_NAME                        BYTES
------------------------------ ----------
T                                 5242880

thiru@9.2.0:SQL>analyze table t compute statistics;

Table analyzed.

-- User_tables shows 122 used blocks now

thiru@9.2.0:SQL>select table_name,blocks from user_Tables where table_name='T';

TABLE_NAME                         BLOCKS
------------------------------ ----------
T                                     122

-- Lets reset the High water mark.

thiru@9.2.0:SQL>truncate table t;

Table truncated.

thiru@9.2.0:SQL>analyze table t compute statistics;

Table analyzed.

thiru@9.2.0:SQL>select table_name,blocks from user_Tables where table_name='T';

TABLE_NAME                         BLOCKS
------------------------------ ----------
T                                       0

-- Now there is no user data in the table. Zero used blocks.

-- Still the initial allocated size remains the same

thiru@9.2.0:SQL>select segment_name,bytes from user_Segments where segment_name='T';

SEGMENT_NAME                        BYTES
------------------------------ ----------
T                                 5242880