50 liters is the smallest size pack most ultra lighters will take on a through hike. 40 is too small for everyone except UL experts with a 5 lb base weight. 60-70 is needed if you are using some combination of an "ultra light" traditional tent, synthetic sleeping bag, or bringing a fleece jacket – none of which are recommended by most through hikers :).
In the spring and fall, you'll have winter-ish gear and 50 will seem too small, and you'll have to strap lots of stuff to the outside.
In the summer, 50 liters is a little big, but compresses to 40 with the side straps easily. Also, you'll want more room for food then since your metabolism goes up a lot after 6 weeks of constant walking. Plus, you'll want to be buy things like bagels, tortilla shells, jars of peanut butter, and bags of Fritos – which need extra room to carry :)
Most of the weight in a pack is in the padding and (to a lesser extent) the frame. The extra material to go from 40 to 60 liters is only about a 1/2 a yard. Look up material weights for 210 denier ripstop with dyneema grid. You'll be suprised.
Personally, I also find that having to carefully fold/roll and pack everything like I'm playing a game of Tetris gets old fast. I like a pack where everything fits in easily, but the pack has enough side/bottom/top compression to make it into a solid bundle.
Also, there is a reason the packs like the ULA circuit (etc, etc) are so popular for through hikers. I'd think long and hard before straying from that volume, amount of padding and frame/support.
