Archive backups still very large?

[Dave]

I'm running a full backup nightly, and then archive log backups every
five minutes (though the backup appears to take 20m). The archive log
backups are still quite large, even with no activity on the database.

Here's some output from pg_rman show:
2013-05-25 19:25:01 20m ---- ---- 100MB ---- 6664MB OK
2013-05-25 19:00:01 20m ---- ---- 83MB ---- 6660MB OK
2013-05-25 18:35:01 20m ---- ---- 100MB ---- 6664MB OK
2013-05-25 18:10:01 20m ---- ---- 100MB ---- 6664MB OK
2013-05-25 17:45:01 20m ---- ---- 100MB ---- 6664MB OK
2013-05-25 17:20:01 20m ---- ---- 100MB ---- 6664MB OK
2013-05-25 16:55:01 20m ---- ---- 100MB ---- 6664MB OK
2013-05-25 16:30:01 20m ---- ---- 83MB ---- 6660MB OK
2013-05-25 16:05:01 20m ---- ---- 100MB ---- 6664MB OK
2013-05-25 15:40:01 21m ---- ---- 452MB ---- 6747MB OK
2013-05-25 13:30:01 129m ---- ---- 4462MB ---- 7698MB OK
2013-05-24 15:25:01 1320m ---- ---- 31GB ---- 13GB OK
2013-05-24 15:08:51 7m 560MB ---- 1224MB ---- 233MB OK

I started up a clean database, then did a full backup. Then I restored
a large dump file (with pg_restore). After that large archive backup,
there was no further writing on the database. Yet every archive backup
has the 100MB of WAL and 6.6GB of "database" backup. What is this?

Here is the backup.ini put in the rman backup catalog for one of these
archive backups:
# configuration
BACKUP_MODE=ARCHIVE
WITH_SERVERLOG=false
COMPRESS_DATA=true
# result
TIMELINEID=1
START_LSN=9/e0000020
STOP_LSN=9/e4000048
START_TIME='2013-05-26 13:45:01'
END_TIME='2013-05-26 14:05:58'
RECOVERY_XID=10332
RECOVERY_TIME='2013-05-26 14:05:53'
TOTAL_DATA_BYTES=35071063473
READ_DATA_BYTES=34514027506
READ_ARCLOG_BYTES=100663615
WRITE_BYTES=6661742689
BLOCK_SIZE=8192
XLOG_BLOCK_SIZE=8192
STATUS=OK