The original 'bergans' were A-frame rucksacks made by 'Bergan of Norway' and issued to Commando and Airborne Forces, SAS and units and formations involved in Arctic and Mountain Warfare. These people got an issue rucksack because they were much better for load carrying than the issue '37, '44 and '58 pattern large packs. Nobody sensible has yet been able to explain what the advantages of 'large packs' for other formations were.
In the 1970s, the rucksack GS and the rucksack SAS began to be issued as above, but also to units conducting rural patrols in NI. These rucksacks featured a butyl nylon sack mounted on an external GS manpack frame which was also used for Clansman radios and ECM equipment. The GS rucksack had a capacity of about 60 litres whilst the SAS rucksack could hold about 120 litres. They carried the weight high on the back above the '58 pattern kidney pouches.
After the Falklands War, where transport problems led most soldiers to carry all of their equipment, most of the time; the Army began to get sensible about rucksacks. A large number of Berghaus and Karrimor rucksacks were purchased 'off-the-shelf' whilst a new infantry rucksack was designed, based on (read stolen from) the commercial 'Crusader' design. This was followed a few years later by the smaller 'other arms' rucksack, AKA the 'REMFs Handbag'.
A significant problem with big bergans is that soldiers stuff them with all kinds of unneccesary bollocks which they never actually use, and the trend now is towards intermediate sized rucksacks, like the superb Infantry Patrol Pack which is now on limited issue.