Tape Size

And to answer your other point ...

The tapes are not configured with a capacity, there is no command to check because it does not exist.

NBU has no understanding of tape capacity.  the different densities, hcart / hcart2 / 4mm etc ... have no technical meaning, they are just a label to allow a certain tape to be used in drives with the same density (eg. hcart tape will only go in an hcart drive).

I can set my LTO drive on my test server, and my tapes as 4mm.  It will (and does ) work exactly the same as if I configured it as hcart, or hcart3 - hell, I could even configure it as 'Micky Mouse', NBU wouldn't care - ok I'm joking on that last part, but you get the idea ...

Tape capacity is determined by the tape drive itself.  At some point near the physical end of the tape there is a 'special mark' that is detected by the drive firmware, and this sets a 'flag' in the tape driver.  When NBU tries to send another block of data the tape driver refuses to accept it (well, the OS does ...) and it passes a message back to NBU that the tape is full, and we need a new one.  So, without this message, NBU would try and write to the same tape for ever.

Reasons for low capacity are usually that the data isn't as compressible as you think it is.  Other causes are bad firmware and even faulty hardware.