Anny Konetzni's middle voice sounds oceanic even in the live recordings. I would add Traubel, Farrell, and Crespin to this list as well.
Thanks for the Claramae Turner. Didn't know her. Sutherland's middle voice became bigger and rounder after her mid 40's.These were all the first examples I thought of as well. True Wagnerian soprani tend to have middle voices which evoke the sounds of mighty ocean waves crashing against the shore on a stormy night. Traubel probably comes to mind as the biggest middle voice of any recorded singer.
other examples
Joan Sutherland
Oralia Dominguez
Jerome Hines
Claramae Turner
Mykola Kondratyuk
her middle doesn’t strike me as immense at all, dark, yes, but artificially so, there aren’t the overtones for the voice to sound as big as most of the others mentioned here. Pankratova seems to be a similar case.And Tatiana Serjan once more.