Backing up large Filesystems

Hello Amanda-Users,

maybe this is a very newby question, but i am running amanda 2.4.4 to dump several filesystems to a DLT Tape.
First backup went fine, but amanda seems to get trouble with the large filesystems. One of them /var is about
70 Gig, but only 4% of it about 2.5 GB, are filled with data. What ist amanda calculating to determine whether a backup fits on the tape.
Calculates it the overall size of the Partition or the actual amount of data on that partition ?
I include an exerpt from the error mail amanda sent me:

FAILURE AND STRANGE DUMP SUMMARY:
www.XXXXX /usr lev 1 FAILED [dump larger than tape, -1 KB, skipping incremental]

STATISTICS:
                          Total       Full      Daily
                        --------   --------   --------
Estimate Time (hrs:min)    0:02
Run Time (hrs:min)         6:20
Dump Time (hrs:min)        6:14       0:07       6:07
Output Size (meg)        3395.1     1701.8     1693.3
Original Size (meg)      4813.9     2500.1     2313.8
Avg Compressed Size (%)    62.7       68.1       52.2   (level:#disks ....)
Filesystems Dumped            4          1          3   (1:3)
Avg Dump Rate (k/s)       154.8     3884.7       78.8

Tape Time (hrs:min)        0:11       0:06       0:05
Tape Size (meg)          3395.1     1701.8     1693.3
Tape Used (%)               8.9        4.5        4.4   (level:#disks ...)
Filesystems Taped             4          1          3   (1:3)
Avg Tp Write Rate (k/s) 5240.3     4589.4     6111.4

USAGE BY TAPE:
Label        Time      Size      %    Nb
tape02       0:11    3395.1    8.9     4

NOTES:
planner: Full dump of www.xxxxxxx.xx:/var promoted from 6 days ahead.
taper: tape tape02 kb 3476736 fm 4 [OK]

DUMP SUMMARY:
                                     DUMPER STATS            TAPER STATS
HOSTNAME     DISK        L ORIG-KB OUT-KB COMP% MMM:SS KB/s MMM:SS KB/s
-------------------------- --------------------------------- ------------
lx18027      hda3        1 981520 981520   -- 344:37 47.5   2:057840.0
lx18027      hda5        1   57990 57990   --   20:18 47.6   0:087351.2
www.xxxxxx /           1 1329790 694396 52.2   1:565968.6   2:314609.8
www.xxxxxx /usr        1 FAILED ---------------------------------------
www.xxxxxx /var        0 25601001742633 68.1   7:293884.7   6:204589.4

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