I'm not sure I follow the discrepancy ... Aren't you saying it does the same thing in mini vMac as well as real Mac?

Either way what you are describing is a bug of the early software. I recall this happening a lot back in the day, I would stick in a disk to save which reported plenty of space to save only to find out there was not enough. Ocassionaly, the opposite would happen, the application would save the file with apparently just enough room, then when you tried to access the disk again, you could not as the disk did not have enough space to save the Finder data, because the file size was not correctly determined, the Finder essentially oversaved, and so the disk could not be read by the Finder. If hou were lucky you would figure is out when you went to shut down as the Finder would alert you to this and sometimes you could put in another disk if you had an external drive and copy your files. But before shut down command, you were usually screwed. Also, there were plenty of applications which assumed things that simply were not true, like having the system on the application disk (Apple's were some of the worst offenders, possibly because that's how Jobs wanted it, or more likely because Jobs was forced to ship software before it was ready thanks to his mercurial tampering).