Great question — answering as the UphillTech team:
That 60 l figure is a middle ground. There are ultralight backpackers who could use a lot less, but that means spending huge amounts on minimalist kit, and refining your setup over time. I come more from the "be prepared" end of the scale, partly because I live somewhere with very changeable weather, and have a very old but very nice 60-75 l pack. That means it expands upwards, making it rather tall at the biggest size. I used it a lot trekking in groups of 2-3, with an appropriate size of tent, and one stove per group. With up to about 3 days between resupply stops, this was plenty, even when we had to re-share the load within a group of 3 to help someone who was struggling. Sleeping bags...
Full answer by Chris H: https://outdoors.stackexchange.com/a/25099 (CC BY-SA 4.0).
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