SYates is correct (as usual)...sort of... The Osprey rucksack weighs as much as it does empty because it is an internal frame pack with an integral padded waist belt. The Osprey waist belt design is among the best in the industry.
The weight of a rucksack is relative to the quality. While heavier bags are not necessarily the best bags, it is true that the lightest, frameless bags, are NOT advisable for longer treks, carrying more than a couple of kilos. At both ends of the weight curve, most rucksacks are likely not suitable to purpose (Camino).
The lightest bags are more suitable to casual hikes and short day hikes. The heavier bags are more suitable to expedition and through hiking use. Conversely, the typical Camino profile can be likened to Alpine "hut hiking" from one shelter (e.g. albergue) to the next hut or shelter for the night.
Thus, the "sweet spot" if you will, seems to be for internal framed rucksacks of medium volume. You will find a LOT of opinions on what the "best" size is for a rucksack. These range from about 24-25 liters, up to 50 liters or more.
In my opinion and experience, the correct range for the largest tranche of pilgrims is likely between about 35-38 liters to about 45-46 liters. But, your experience and needs may well differ. However, if you are just starting out looking for a Camino rucksack, and are a "normal sized" person, consider this as the place to begin your search.
But, over years of experience, experimentation, and after many sore shoulder and back muscles, and IMHO, and generically stated, the "best" rucksack solution is the one that:
- carries the stuff you decide YOU need for YOUR Camino,
- in a manner you determine best (all inside, versus having "dangly bits" strapped to the outside),
- protects the contents from rain, snow, or casual "appropriation" by others during rest stops,
- in a comfortable manner over the span of the day, in all weather. (PERIOD - END STOP)
Another consideration is that larger people (like me) wear clothing that is necessarily larger than smaller people. This results in a net increase in pack volume required to "house" all this gear. So, a petite woman wearing XS or S sized clothing will obviously require a smaller pack to contain her extra clothing than will someone who is literally more than twice her size (me), wearing clothing in the XXL range.Trust me, I DO envy pilgrims of smaller stature who can walk a Camino with a 34 liter pack.
The weight penalty for the smaller person also tends to be less, as non-clothing gear items tend to be one-size and weight. So, the poncho I carry will be the same as the poncho a much smaller person might carry. Similarly, my smart phone charger and cable is more or less the same as all others...etc.
So much for overall volumetrics...returning to empty pack weight...
Even though the vertical stays, or ribs, are usually a durable ABS plastic, they must have a specific mass and weight to do their job of providing support and rigidity. This goes with having a quality rucksack, of any brand. Some manufacturers still use aluminum stays. These can be better in some circumstances and do bend to conform to the curve of your back. However, they tend to weigh more than the plastic varieties.
Even if you compare this rucksack (Osprey) to other major makers, such as Deuter, Ferrino, the North Face, and many others, you will find they use pretty much the same support system. The biggest differences are usually in the back support panel system.
Some makers use a ribbed pad, alone, or under a webbed piece of fabric to enhance circulation. Others use a stiffly suspended piece of ventilated fabric in a trampoline sort of arrangement to suspend the pack from your back to provide a cushioned carry and ventilation. Some merely use a pad, covered in ventilated fabric.
Some of these choices are cost-driven. Others are performance-driven. In the end, the weight variance among the better rucksacks comes down to grams or a few ounces. What matters most, at least IMHO, is how the pack feels after 10 or 20 Km, over undulating or hilly terrain, in a range of weather.
I do believe that, after a few days on Camino, the "best rucksack" for you is the one that fits like a comfortable pair of old shoes or your favorite sweater (pull). You feel good hoisting it onto your back in the morning, and actually feel somewhat "naked" when you remove it at breaks and at the end of each day's walk.
THAT is how I know my Osprey Kestrel 48 liter pack is right for me. I need the volume to carry the stuff I require. This includes clothing, gear, and the extra nutritional supplements and medication I must carry for personal reasons. The back panel conforms perfectly to my back, shoulders and hips, the weight is relative, and I DO feel somewhat naked without the pack after a day's walk.
I hope this helps the dialog.