I find that really interesting and not logical at all...here is why. Say when you are upgrading a firewall (just as an example), if you do verify, it would tell you why upgrade is not possible, whatever the reason. You would think same would happen here, but I guess not. Though, I could have sworn that few years ago when I tried it for one customer, it was complaining about lack of space, but this was definitely not the same version as yours and it was not included, we had to download it, so its possible there were differences.
What was again the type of appliance? VM?
Andy