I'm really not familiar with pack volumes; I just looked that up on a website, which listed normal packs as 40-60 liters, ultralight 30-40, and super ultralight <30.
I don't know how big my old pack was, but it was about the right size for me. Unfortunately, while it's frame is still in fair shape, its fabric is shredding badly. It was a real bit of patchwork just to keep it together last time. We're talking "stuff falling out" bad.
I'm used to carrying everything I need. I've done dry camps before. In Japan, I averaged carrying around 60 pounds with a bulging pack. That trip was a bit annoying, gear-wise, because I had to bring lots of clothes (so as not to fall into the stereotype of a "dirty foreigner" -- they're very obsessed with cleanliness). It made for a lot of bulk -- and when things got wet (from the near daily rains), weight. My partner did have a bit of trouble with 50, though, so we'll need to keep hers lighter.
How well do the Osprey and MEC open up? With my old pack, it was just a zipper for the bottom third of the pack at the bottom and a typical flap opening at the top -- certainly not easily accessible.
I'm not too concerned about food volume. On the Japan trip, I discovered a great way to pack lots of calories into little space: pasta and sauce/rice and sauce packs like you get at the grocery store. I'm not sure if you're familiar with these things, but they're a mix of dry rice or noodles and a powdered sauce, in a wide variety of flavors (everything from cajun to chinese), coming in either little cardboard boxes or aluminized paper pouches. They're designed for you to cook them for a few minutes with butter, milk, or a little bit of other stuff, plus (of course) water. They taste really good -- usually far better than backpacking meals. However, their dry volume is just as small as backpacking meals, if not smaller, and they're far cheaper. Take them out of their bulky packaging, put them into small ziplock bags with the proper amount of powdered milk, butter-flavored powder, or whatnot, and then load the individual meal-bags into a gallon bag. I was very pleased with how that turned out; I was able to fit 30-ish, possibly more, into the gallon bag. Not sure of their calorie content; at home, 1/2 usually makes a good lunch, and 3/4 a good dinner. While hiking, I found myself eating about one for a lunch and two for a dinner. So, that works out to 3-4 days worth of lunches and dinners per liter of pack space. Say, 2 days when you factor in breakfasts, snacks, and overhead. 1-2 weeks out there, that's 3 1/2 to 7 liters of space needed for food per person. Not too bad.