So, Jim, size doesn't matter?

Actually, I agree. I recently selected an Exos 58 over an Atmos 50. Both packs would hold all the gear I'd ever carry, but the Exos does so more easily, and my sleeping bag and down clothing are less compressed. Also, I can use the various compression straps to shrink the size if necessary. So, like Jim, I'm a fan of larger packs. (Besides, the Exos weighed almost a pound less than the Atmos.)

Like another poster said, if you're just getting into ultralight, a smaller pack may help you resist the urge to take stuff you don't need. However, like Jim says, it may also force you to compress gear. It may also trick you into taking gear that is too small - like a pot that is too small to cook with conveniently, a stove that (bacause the supports are small, and the base too narrow) results in a too-tippy setup, a pad that is too short for comfort and too thin for warmth, and so forth. But hey, it fits in the pack and makes you look cool, right?

For what it's worth, my cold weather load for a weekend is only 21 pounds, mostly because manufacturers are doing a great job of making truly innovative, light gear - so much so, that you have to watch closely to make sure you get what works for you. So I'm not knocking hiking lighter. I just think smarter is as important as smaller.