Unless this tape was created by using Object Copy, you should not be using 'Import As Original'. Export it, and import it again without this argument.
Having said that, the listed capacity really doesn't matter, what is important is that all the objects were imported. This can be determined by running
omnidb -session [session_id] -detail Don't include the [ ] marks
Frankly, I am surprised, normally, when you import tape, the size is shown as '0 kb'.