I do not tour full time, but I just got home from a 5 week tour.

For a decade I have been using the Ortlieb 40 liter rear and 25 liter front Ortliebs. Sometimes I added the 31 liter duffle on top in back.

But several years ago I bought a pair of Carradry rear panniers, they were rated at 50 something liters. I do not recall the rating but I think they are about 50 to 52 liters which is much smaller than the rating was. I used them on my most recent trip instead of my Ortliebs to see how they work. Plus they did lower my center of gravity a bit. That larger volume meant that I could put a lot less volume on top of the rack, instead of the 31 liter duffle, I had a dry bag that varied in size from maybe about 8 to about 20 liters depending on how recently I was at the grocery store.

I can't possibly imagine needing 70 liters. But if you were going across a desert and needed to carry lots of both food and water, then maybe you have a specific need. At times I have carried up to two and a half weeks of food, but I never needed to carry more than 5 liters of water.

In the photo I have about two and a half weeks of food on the bike with my 40 liter Ortliebs, 25 liter front Ortliebs, the 31 liter Ortlieb duffel (rack pack?) and an extra blue drybag for over flow food.

If you decide to get the 70 liter bags, you will need a very stout bike to carry that much of a load without handling like a wet noodle. The bike in the photo is rated to carry up to 60 kg not counting the weight of the rider, it could handle the load you are talking about but not many other bikes could do it.