After checking the current size of my XE instance in my yesterday's blog I decided to take a closer look at SYSTEM and SYSAUX tablespace:

SELECT tablespace_name
     , SUM(bytes / 1024 / 1024) size_mb
  FROM dba_segments
 WHERE tablespace_name IN ('SYSAUX', 'SYSTEM')
 GROUP BY tablespace_name
;

TABLESPACE_NAME                   SIZE_MB
------------------------------ ----------
SYSAUX                           764,4375
SYSTEM                           434,6875

SELECT segment_name 
     , segment_type
     , bytes / 1024 / 1024 size_mb
  FROM dba_segments
 WHERE tablespace_name = 'SYSTEM'
 ORDER BY bytes DESC
;

SEGMENT_NAME                   SEGMENT_TYPE          SIZE_MB
------------------------------ ------------------ ----------
SOURCE$                        TABLE                     143
IDL_UB1$                       TABLE                      42
IDL_UB2$                       TABLE                      15
C_OBJ#_INTCOL#                 CLUSTER                    11
C_TOID_VERSION#                CLUSTER                    10
PK_C0                          INDEX                      10
PK_O                           INDEX                      10
PK_CT                          INDEX                      10
PK_CS                          INDEX                      10
ARGUMENT$                      TABLE                       7

SELECT segment_name 
     , segment_type
     , bytes / 1024 / 1024 size_mb
  FROM dba_segments
 WHERE tablespace_name = 'SYSAUX'
 ORDER BY bytes DESC
;

SEGMENT_NAME                   SEGMENT_TYPE          SIZE_MB
------------------------------ ------------------ ----------
SYS_LOB0000010794C00025$$      LOBSEGMENT                 58
WWV_FLOW_PAGE_PLUGS            TABLE                      30
WWV_FLOW_STEP_ITEMS            TABLE                      27
I_WRI$_OPTSTAT_H_OBJ#_ICOL#_ST INDEX                      22
WRI$_OPTSTAT_HISTGRM_HISTORY   TABLE                      14
WWV_FLOW_REGION_REPORT_COLUMN  TABLE                      12
SYS_LOB0000012406C00009$$      LOBSEGMENT                 12
WWV_FLOW_STEP_PROCESSING       TABLE                      11
WWV_FLOW_PAGE_PLUGS            TABLE                      11
SYS_LOB0000012186C00017$$      LOBSEGMENT                 11

After reading in the OTN thread Does tha UNDO Ts count on Database Storage Limit ? I tested my own instance using Paul M.s SQL statement:

SELECT total
     , allocated
     , allocated / total * 100 percent_free  
  FROM (SELECT 5120 total
             , SUM(bytes) / 1024 / 1024 allocated  
          FROM dba_data_files  
         WHERE tablespace_name != 'UNDO'
       )
;

     TOTAL  ALLOCATED PERCENT_FREE
---------- ---------- ------------
      5120       1510   29,4921875

SELECT total
     , allocated
     , allocated / total * 100 percent_used
  FROM (SELECT 5120 total
             , SUM(allocated) / 1024 / 1024 allocated
          FROM dba_data_files df
             , (SELECT tablespace_name
                     , SUM(bytes) allocated
                  FROM dba_segments
                 GROUP BY tablespace_name
               ) sg
         WHERE df.tablespace_name  = sg.tablespace_name
           AND df.tablespace_name <> 'UNDO'
       )
;
     TOTAL  ALLOCATED PERCENT_USED
---------- ---------- ------------
      5120  1230,6875   24,0368652