Photographic Dynamic Range is by definition measured at the same normalized size: “PDR is the dynamic range you would expect in an 8×10″ print viewed at a distance of about arm’s length.” https://www.photonstophotos.net/Gen...Primer/Photographic_Dynamic_Range_Summary.htm It is not pixel dynamic range.Unless one is pixel peeping at 100%. Then the smaller sensels mean the image is being enlarged by a greater factor at "100%" viewing and thus noise is easier to see.
But yes, if images are viewed at the same normalized size from the same sized sensor and are from the same manufacturer/generation of technology, they'll have comparatively the same S/N ratio. Obviously in the example you showed, there's some different processing going on between ISO400 and ISO1600.
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