Mountain Backpacks and Trekking Backpacks have useful features to improve comfort and payload on the back; Internal or external structures, rigid or semi-rigid, allow the weight to be distributed homogeneously from shoulders to the basin, exploiting the ergonomics of shaped shoulder straps and the ventral band, well padded and breathable materials.
The volume (ie the capacity) of a Backpack is measured in "liters"; From micro racing bags that have an inside volume of 10/12 liters, to load giants that can exceed 100 liters.
There are no "precise" rules, but you can establish a "Relationship" between the volume and the weight of a Backpack:
Volume Empty weight
Backpack for easy hiking: 25/35 liters less than 1.200 gr
Backpack for daily excursion: 35/50 liters gr 1,400 / 1,700
Multi-day trekking backpack: 60/80 liters gr 2.000 / 2.500
Very important feature of these backpacks is the dorsal ventilation system that must allow a good ventilation of the back.