I guess I have a "Jack Bauer" opinion of a BOB -- it's for bugging out, not camping. 4000-5000 ci is a camping pack. If your idea of bugging out is to head to the hills and live off the land then yeah, you need a big bag. But is that a realistic Bug-Out plan? IMO, the first thing any Bug-Out plan should have is a destination, a place to bug-out to. Size your BOB to get you there and stash a cache of supplies in that location. No need to carry it if it's already there.