Hi,
See there are many reasons for control file growing.Though u dont have any structural changes or backup or archived logs Oracle updates the controlfile and the database files with the checkpoint (SCN).
Also when there are log switches Oracle updates the controlfile with the log sequence number.
In order to trim ur controlfile....change the init.ora parameter CONTROL_FILE_REUSE_TIME=3.by default the parameter is set to 7 days.If u keep the no of days to less than Oracle will keep the information for 3 days and then overwrite it.
In case of any doubts please be free to write to me at [email protected]

Regards,
Rohit Nirkhe,Oracle DBA,OCP 8i
[email protected]