I think the savegame size can be trimmed noticeably by removing character history, pondered about it myself a long time ago. The problem is how to do it to not affect the game significantly, it has to be done selectively.
For a living character whose all ancestors are dead already grandparents and older ancestry can be deleted, as Hardradi suggested. No character who was a ruler should be deleted, and the ruling family should be extempt in monarchies.
Note that generals are listed for their battles too, not sure if the war exhaustion isnt calculated from the recent battles.