Interesting, thanks. What tool are you using to view the EXIF, I don't see anything listed in the options of exifviewers.com labelled "Sensor Width".Nope.
I just did a quick test with R5: those resolution figures change when it's set to lower resolution.
In a full resolution image:
Focal Plane X Resolution : 5773.079634
Focal Plane Y Resolution : 5769.799366
which work out to 45 megapixels as expected.
In a M-quality jpeg:
Focal Plane X Resolution : 4093.023256
Focal Plane Y Resolution : 4088.701162
which work out to 22 megapixels.
So those particular numbers don't prove anything.
As far as I can see, the only field in the M-quality image exif that reveals the sensor is actually bigger is "Sensor Width", which is the same, 8352, in both images. I haven't looked at any of the R3 images in the web to see how it looks in them, but I'm sure somebody will.