@hikermor I have a Traverse 35 also and I'm not sure where ODGL got 48L either. I have a Medium and the main compartment is in the 35-37L range according to my measurements...pretty much on par with what REI states. It depends a bit on how closed the bag is for the measurement. Possible ODGL added all the pockets.
Oddly REI give a another size measurement...M: 26x 12 x 11 inches = ~3500cu inches which is ~56L ...I have no idea what that is a measurement of since it seem unrelated to the pack...except perhaps as a guide for a shipping box...or maybe a max packed size if you want to take it on an airline.
Regardless, it is a good day/overnight/no bear can pack and carries well although I have only tried carrying about 20lbs with it. It has a couple of flaws... the sternum strap sliders are too loose...the wrong size filler cord was used I think...not sure if that was just a batch problem...and the padding on the hip belt padding could come further forward on the hips for better effect...the pads seem an unnecessarily economical length given their somewhat technical construction.