Well, naturally it's that short
The saving routine itself crashes out during accumulation of the data to put into the save. That results in the application terminating prematurely. Windows will then close down the still open save game file, at the point where the save game data had already been committed to disk. Given the standard cutoff size of 59KB, that is presumably only the first commited data block.
In fact, having observed Windows behaviour when writing data to a large file, Windows usually only commits the very first data block, buffers additional multi megabyte data in RAM, which then only ends up in the disk file if either the program explicitly calls flush for the data stream or closes the file. So, the crash could have been near the end of the data stream, and you still end up with a 59 KB file.
Having said all that, I've seen the pure virtual function call crash during saving as well. But I was still on april 5 beta at the time, so it was not opportune to post a report for it. Having said that, at the time of the crash, like the thread starter, in my game a crusade was in effect. Secondly, I was at war with the tribe of Pommerania, and had confiscated all of their provinces. The last province of the tribe was under siege by a third party, and moments before the autosave point I had rejected a peace proposal that was triggered by some event, instead of the usual way (through a diplomat of the opposing party knocking on my doorstep).
Jan Peter