system tablespace's extend

Hi folks:
  I am coming up with a strange problem. my system tablespace keep on increasing every day by 100M. and i query the v$dba_segments ,then i found this table: IDL_UB1$ size is 15031091200bytes.
it have got 114679 extents.
BTW,my sytem data file's size is almost the same as IDL_UB1$. Can some one throw some light on it,that would be appreciate.

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