Some people new to backpacking buy rucksacks that are too heavy for their needs or strength and fitness and this does not lead to comfortable, successful back packing or trekking. Many 65 – 79 litre volume rucksacks with full internal framing and massively padded belts and back weigh 2.5 kilos before putting anything in them. These are often great quality, well made products and if you are going to be carrying several days supply of food and water plus all the camping gear and clothing for an extended trip and you are strong enough to carry 15 plus kilos comfortably day after day they will give good service.
There is a lightweight alternative approach, when carrying large amounts of food and water are not required, that enables more comfortable trekking and bigger daily distances with less fatigue and blisters. When we walked the Camino de Santiago De Compostela I carried a 60 litre pack that weighed 7 kilos when fully laden which included a full size thermarest mat and a tarp and pegs and sleeping bag. My partner carried a 42 Litre Go Lite pack which weighed 5.5 kilos and had all the same gear except the tarp. Children should use the lightest packs - usualy under 20 litre capacity.
The secret of the lightweight was partly due to being ruthless in packing and only using lightweight sleeping bags and clothes and a tarp instead of a tent, but a main factor was the rucksacks which were frameless and lightweight with a minimum of padding and webbing. My 60 litre weighed 600 grammes and my partners 42 litre weighed 400 grammes empty.
Another way of keeping weight down is simply to use a smaller rucksack. I walked and camped the Pembrokeshire Coastal Path carrying only a 35 litre rucksack with my thermarest mat and lightweight tent strapped to the sides of my pack. It worked well because the weight was kept down to about 7 kilos.
These frameless, unpadded rucksacks have to be loaded carefully (see loading your rucksack) or they are uncomfortable with hard things like stoves poking in your back but apart from that they are excellent for lightweight trips where you are carrying less than 9 kilos which in my experience can be any trips where you are not carrying large amounts of food and water. One trick for loading them is to use your foam sleeping mat as a tube/sleeve in the rucksack and to put everything inside that.
Loaded with more than about 9 kilos these ultra lightweight sacks are uncomfortable with their thinner shoulderstraps and unweildy with their lack of structural framing, a bit like carrying a sack of coal roped to your back. So if you think you will be carrying a lot of food and water and a heavier tent then you will need to step up to shouldering the extra 2 kilos of the full on traditional backpacking rucksack which spreads the load evenly and protects your back from uncomfortable objects in the rucksack.
Many manufacturers are now aware of weight as an issue and are making their packs lighter, coming in at various weights with differing amounts of structure and support built into the rucksack.