Think of it the other way: how many soda bottles could you dump into that pair of panniers? Sounds like a big number? It's 1.4 cubic feet or a bit less than an 18" cube.
Bear in mind that capacity doesn't come close to describing utility in terms of packing, finding, unpacking & repacking stuff. Two different containers, each 40L could be vastly different in layout, and thus utility.
pubb