I use the following two scripts to get a list of those big
backup files.
First run script 1 to get a list of the tapes you used during the
last backup run. Then run script 2 for every tape you got from script 1.

1) Get a list of tapes used by last nights backup run (change start and
stop time and list of storagepools)
select volume_name as Volume,last_write_date - 
from volumes where - 
last_write_date > timestamp(date(current_date-1 day),'19:00:00') and -
last_write_date < timestamp(date(current_date),'10:00:00') and -
stgpool_name in ('TAPEPOOL','TAPEPOOL2','TAPE_TDP')

2) Get all files from volume $1 bigger $2 bytes (long running query)
select node_name as Node, -
filespace_name as Filespace,-
file_name as Filename,-
file_size as Size -
from contents -
where volume_name='$1' and -
file_size>$2

Gives you a listing like:
NODE                FILESPACE           FILENAME
SIZE
------------------  ------------------  ------------------
--------------------
server1             SYSTEM OBJECT       \ADSM.SYS\W2KREG-
14833834
                                         \REGISTRY\MACHI-

                                         NE\SAM\DEVICE\H-

                                         ARDDISKVOLUME1\-

                                         WINDOWS\SYSTEM3-

                                         2\CONFIG\ SAM

server1             SYSTEM OBJECT       \ADSM.SYS\W2KREG-
14833834
                                         \REGISTRY\MACHI-

                                         NE\SECURITY\DEV-

                                         ICE\HARDDISKVOL-

                                         UME1\WINDOWS\SY-

                                         STEM32\CONFIG\

                                         SECURITY