The high backuppc data volumes appear to be a problem with File::RsyncP on the Ubuntu port to the ARM architecture.  I have created a small test script that calls File:RsyncP to copy files from one directory to another on the same system.  Running the script the first time copies all the files (about 17MB).  On Linux/Intel, subsequent runs transfer only control information.  On Linux/ARM, a large number of the blocks are flagged as different even though the files themselves are identical.  As a result, 7.5MB of the file data is transferred.

I have send the results of the test to Craig Barratt and also posted to the ubuntu-devel-discuss mailing list.