Ortlieb used to make --edit -- still make, the XL ones. 35L x 2 = 70 litres.
I saw them in a store years ago and you could see that for a cold weather trip with bulkier warm clothing , sleeping bag etc items, they would be great.
I've never seen them in stores again, as I imagine they do not sell that many of them. They were and are a lot more expensive than regular 40L ones, so not a big incentive, but mainly I see them as being a real possibility of just plain overloading and very likely having a pretty waggy steering bike.

It seems to me that Arkel had a much bigger volume model of rear panniers also, not sure if still made.