Be realistic about your potential emergency and then prepare for it.
I have a BOB in each of my cars and they are no larger than a ladies purse and the item that takes up the most space is 3 bottles of water. In my area there is no place where I can go that I can't crawl (not walk but crawl) out of in one day to get help... usually a road.
If you are truly in an emergency situation who's going to worry about wiping your ass?!?!?! Why pack a GPS plus spare batteries when you can pack a small compass for much less space and weight while serving same purpose? Why carry a tent, rain gear and poncho when a large pancho can be used for all three purpose?
More than anything else you need knowledge and common sense... which is not so common these days.
Most people will never be in a place where it takes days to walk out to a road for help.