For backpacking, I leave my tent, sleeping bag, pad, stove and then maybe a few things like extra clothes and thing I don't need/aren't concerned about. Everything else like all food, emergency stuff, outer layers, etc. goes with me in the day pack. Once you get all that other stuff out of the pack, I find it's pretty easy to carry everything else, especially if you're using your backpack compressed down as a daypack which is what I do. If everything I left in camp were to disappear, be it by wind or animal or man, I could survive with what I have in my backpack. That's my rule of thumb. I never leave my minimum to survive anywhere when I'm in the backcountry. It stays with me.
As far as actual theft, I've never been too concerned backpacking but largely because I don't leave much 'grabbable' stuff. It'd be kinda tough to steal an unpacked tent, pad and bag, IMO. I remember reading on BCP a while back that someone's friend left a handgun in a tent up at Red Castle and it was gone when they returned.
For car camping, I totally assume that someone may toss our stuff and take things so I just don't bring as nice a stuff when I car camp. I have a dedicated car camping tent, pad, bag, etc. It'd suck if it went missing, but I wouldn't be out $1000+ like my backpacking setup.