Dyson were unique in their washing...
I can see why they needed a 96litre drum to wash 7kg, nothing to do with wash time, but their wash action. I don't think you can stuff the machine to the brim and expect it to wash as it should, as the clothes needed to move freely to enable the 2-drum wash action. I've never owned one but seen videos, and I think if the machine was full you wouldn't get that hand wash action. But wow, 78 litres in the 7kg Dyson was bigger than Candy's 10kg, you could get away with more than 7kg though.
Dyson...well, they were expensive, there were two motors going wrong in the two drummed machine, they looks alone put people off buying them, which is why they failed. I quite liked Dyson's wash action, still do as it's more effective than most machines nowadays, it was a bit like Hoover New Wave of the 90's as that was a machine which was most ahead of its time (and still is) with the wash action.