Today I saw a pack at REI that looked a LOT like my dyneema Smart Alec in form (REI Rucksack 28), with effectively a built-in upper module (which I don't have) as the top of the bag. It said 28L size, but it looked smaller than my SA. Curious, I got it and packed is AS FULL as possible (and I am a TIGHT packer, no airspace left unfilled). Then I transferred it to my Smart Alec. It ALL fit (ok, 1 water bottle didn't, but that's not fair as both water bottles stick out the sides of the REI bag, and I did somehow shove the Hydroflask into the intended side pocket)). So somehow I got EVERYTHING into a, by all rights, 2L smaller bag.
Curiosity peaked, I tried loading my Osprey Radial 34 which I've been using for riding to school and not being COMPLETELY sweaty before class (not a good look for the prof really). A few tiny empty spaces to fit REALLY small things (like chapstick) but otherwise, STUFFED (in a bag that 8L bigger).
Kiddo's REI 35 traverse was the only bag that could take what the SA had (and a bit more, but come on, at that points it's SUPPOSED to be friggin 9L larger, and it wasn't by all THAT much).
TB bags, and especially those with halycon (which is still dyneema to me), really are magical bags of holding that just SWALLOW gear. I wondered how the heck I'd made it through 12 days in Italy with just my SA, and now I know.
So if you were wondering if the holding capacity is really is what people claim, it really is