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It wouldn't make that much of a difference at the image level.In good light, mu43 can hold it’s own. Low light became an issue. The 61 mp Sony FF sensor has a 26 mp APS-C crop. Mu43 is about half the size of APS-C, so figure the 12 mp sensors have similar pixel sizes. It’s interesting that mu43 never tried a newer tech 12 mp sensor, which might have had much better low light behavior. Sony did that with the A7S series for 12 MP FF sensors. Mu43 could have had 12 and 20 mp models.
Larger megapixel counts mean fewer photons per pixel, but once you downsample the larger mp image down to the smaller mp count - or display at the same absolute size - to make for a fair comparison, the only photons you've lost are the ones that bounce off the "walls" between the pixels. BSI technology (which current consumer m43 cameras don't have) and microlenses on the sensor can help further reduce the number of bouncing photons.
The reason video centric sensors lile the GH5S use lower pixel counts are 1) because they don't need any more given a video resolution standard (HD, 4K, 8K etc), and not having to downsample your sensor output saves processing power, heat etc; and 2) fewer pixels makes for faster readout and less rolling shutter with electronic shutter, which is important for video.
The Sony a7S really solidified the thought in peoples minds that the lower resolution allowed for much better low light results at image level (as opposed to "just" at pixel level). But that insane low light capability was just because it was Sony's first Dual Gain sensor. The high res a7R series also got Dual Gain sensors from the Mk II onwards, and perform about as well as their a7S contemporaries in low light - simply because the total number of captured photons is what matters, not the photons per pixel.