Very true. And side by side that aspect cannot be missed. Fortunately, my room is in a basement with no external illumination so I always have the nits very low and the gray of the SD not really noticeable. Even set to 500 nits in a dark room is like looking into the sun. My XDR is on the P3-500 nit preset and the brightness slider is only about 1/3 full.
The HDR is another significant difference (as XDR in its name implies). Try opening a test page like https://www.wide-gamut.com/test or https://www.wide-gamut.com/test/image-hdr in Chrome on both displays—just be sure to use the "Pro Display XDR (P3-1600 nits)" preset. (When it comes to displaying SDR, it's very true they have equivalent visual quality ignoring their size difference.)
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