EXPORT TO SERVER differences in sizes of moved data ?

[Zoltan Forray/AC/VCU]

I just moved a node from an MVS/zOS TSM server we are phasing out, to our
AIX TSM server.

I am wondering why the occupancy sizes are different. The file counts are
the same.

MVS TSM server  (v5.2.4.2)

Backup Files    1,027,310
Backup Size     383.2GB

Archive Files   732,454
Archive Data    39.5GB


AIX TSM server (v5.3.1.3)

Backup Files    1,027,310
Backup Size     377.7GB

Archive Files   732,454
Archive Data    39.3GB


Difference      422.8GB (MVS) vs 417.0GB (AIX)

I have 6TB to move and am a little concerned about these "losses" !