Im curious what type of bug causing crashes like this? It doesnt depends on file size, and surely not on progressing (quest, etc).
The only solution is that the game does something wrong with the saving. Maybe a slow filling buffer or memory leak, written into the save, causing the crash (like you always multiply the nr of the AIs on the map, sooner or later it will crash)
Its a bad news then, because if they fix the bug, the currupt save will remains in the same state.