Symptoms

The tape size under Acronis Tape Management is smaller than the tape capacity specified on the tape box.

Solution

Most tape drives now include the automatic data compression that is performed during the copy-to-tape operation. Because of that, there are two capacities values that can cause some confusion: 

1. The raw or native capacity is the "real" capacity of the tape that it can fit without any compression. 
2. The compressed capacity - this is the native capacity multiplied by 2. This capacity is usually shown on the tape box. It assumes a 2:1 (2 to 1) compression ratio after the data is copied to tape but there is no guarantee that the compression ratio will be 2:1. It can be higher like 3:1 or smaller or even with no compression if the data that is saved to the tape is already compressed (for example the Acronis backups).

   Capacities are often stated on tapes as double the actual value; they assume that data will be compressed with a 2:1 ratio (IBM uses a 3:1 compression ratio in the documentation for its mainframe tape drives. Sony uses a 2.6:1 ratio for SAIT).

Also please note that the units for data capacity and data transfer rates generally follow the "decimal" SI prefix convention (e.g. mega = 10^6) and not the binary prefix convention (e.g. mega = 2^20).

Acronis software shows the raw/native tape capacity because it uses its own default compression and the backups cannot be compressed further.

The information about the HP tapes capacity can be found on HP support center.

Additional information about the tape sizes for different kinds of tapes can be found on Wikipedia