I tend to prefer dayhiking, too. And, I have back problems. I have a Kelty Hawkeye fanny pack: 800 cubic inches which does appear to be bigger than the ones you mention. I'll also then hang stuff (mainly my camera) off the front belt. For a larger pack, have you tried an external frame? If you set it up right, you can put all the weight on your hip belt, and just use the shoulder straps for stabilization (keep the pack from falling backwards). The external frame is just fine on my back, while whenever I try an internal frame it puts way too much weight on my shoulders and I cannot find a way to fix that.