The problem here seems to be that the encrypted file was never properly encrypted fully – as you note, the encrypted file is much too small. Even though AxCrypt does perform compression, a .avi-file is already compressed and will not compress at all or very little. So the expected size of the encrypted file would be in the same range as the original file, i.e. 599Mb.
So, something must have happened to interrupt the encryption process. In this case, the original file should not have been wiped either. Is this perhaps a file that was encrypted some time ago, so whatever happened happened so long ago you don’t remember the details?
In any case – there’s no way to decrypt data that is simply not there. Sorry.
I don’t quite understand. If the file was on an external drive, and it was disconnected – that would certainly explain why the encrypted file is damaged. But not how the original file was deleted. If it was on the same disconnected drive (and AxCrypt via the UI does not support encrypting to a different drive), the original can’t even in theory become deleted by AxCrypt – the drive is no longer connected!
If you did re-connect the drive, and then manually deleted the orginal, that of course would explain it.
And, sorry, no – there’s no way we can extract 599Mb from 16Mb…. With try broken file you should be able to recover the first aprox 16Mb, but that’s only about 2.5%…
If you actually did manually delete the original, there may be a chance that a regular undelete utility can recover it from your drive, provided you’ve not used the drive much or at all since the deletion.