First of all, make sure that
SYSTEM tablespace is not assigned as default or temporary tablespace for normal database users.
Second of all, one of the space consuming table in
SYSTEM tablespace is
AUD$. Next, as you have said you are just performing export/import, datapump creates master tables in the default tablespace of the user performing export job.
The master table is either retained or dropped, depending on the circumstances, as given here. You might have done the export using
SYS user. If the job is failed master table will not be dropped.
Now, you can use the following query to see which object is consuming most of the space on this tablespace.
SELECT owner, segment_name, segment_type, extents, blocks, bytes
ORDER BY bytes;
If you need the audit trail then copy its records to another table, another tablespace if needed, and/or export the audit table, otherwise you can truncate it.
sqlplus sys as sysdba
SQL> CREATE TABLE backup_aud$ AS SELECT * from sys.aud$;
SQL> truncate table aud$;
You can moveaudit trail table, Create new tablespace
Move the audit trail tables using procedure
SYS.AUD$, to another tablespace using
audit_trail_type => DBMS_AUDIT_MGMT.AUDIT_TRAIL_AUD_STD,
audit_trail_location_value => 'NEW TABLESPACE');