depending on what modules you are running it could be a number of things. I would have a look inside the UCS and see what the biggest files are in each, it should you obvious straight away where the discrepancy is. The SOL below has more info on this procedure...
tar xzvf /shared/backup.ucs -C /shared/SOL12278/
find /shared/SOL12278/ -xdev -type f | xargs du | sort -rn | head -5
93740 ucs/var/tmp/filestore_temp/files_d/Common_d/epsec_package_d/:Common:epsec-1.0.0-343.0.iso_39857_1
38728 ucs/var/tmp/filestore_temp/files_d/Common_d/epsec_package_d/:Common:epsec-1.0.0-321.0.iso_34752_1
18440 ucs/var/tmp/filestore_temp/files_d/Common_d/sandbox_file_d/:Common:hosted-content::BIGIPEdgeClient20131211-03.exe_1386755520672_30169_1
18440 ucs/var/tmp/filestore_temp/files_d/Common_d/sandbox_file_d/:Common:hosted-content::BIGIPEdgeClient20131211-01.exe_1386754937182_30166_1
18440 ucs/var/tmp/filestore_temp/files_d/Common_d/sandbox_file_d/:Common:hosted-content::BIGIPEdgeClient20131211-00.exe_1386715165319_29750_1
cheers