archive log files - "flaky" sizes

My archive log files show period inconsistent sizes..Can someone explain whats going on here? (No - there were NO "alter system switch log files" issued during the below period. Nor were the log files altered in the below period. So that's definitely not it).

See below : the logs are sized at 500M. But noticed the archive logs on "Nov 19 18:39" or "Nov 19 21:07" or "Nov 20 00:01"..the sudden deviation in sizes..
Whats going on here? (Yes..the last one was "switched" after yesterdays backup)

SQL> select * from v$log;

GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS
---------- ---------- ---------- ---------- ---------- --- ----------------
FIRST_CHANGE# FIRST_TIME
------------- --------------------
1 1 6995 524288000 1 NO CURRENT
410955998 11/20/02 11:04:53

2 1 6994 524288000 1 YES INACTIVE
410943739 11/20/02 09:12:06

3 1 6993 524288000 1 YES INACTIVE
410747781 11/20/02 05:01:38



NUP-bull1:oracle[bs1]/u06/oradata/archive> ls -ltr | tail
-rw-r----- 1 oracle dba 524289024 Nov 19 20:56 1_6985.dbf
-rw-r----- 1 oracle dba 524286976 Nov 19 21:07 1_6986.dbf
-rw-r----- 1 oracle dba 524285952 Nov 19 21:45 1_6987.dbf
-rw-r----- 1 oracle dba 524289024 Nov 19 23:05 1_6988.dbf
-rw-r----- 1 oracle dba 524289024 Nov 19 23:10 1_6989.dbf
-rw-r----- 1 oracle dba 524281856 Nov 20 00:01 1_6990.dbf
-rw-r----- 1 oracle dba 524289024 Nov 20 02:40 1_6991.dbf
-rw-r----- 1 oracle dba 524289024 Nov 20 05:03 1_6992.dbf
-rw-r----- 1 oracle dba 524289024 Nov 20 09:14 1_6993.dbf
-rw-r----- 1 oracle dba 144038912 Nov 20 11:05 1_6994.dbf