Once again, you are correct given the context you specify. Thanks.
Cropping to the same number of pixels will result in a different field of view. The same bird on the same branch with the same illumination, larger pixels will have less visible noise when cropped to the same number of pixels and the field of view will also change. (so the image will be cropped differently to get the same number of pixels and attempt to make the image pleasing. Two different images. No attempt to get the same image from two different cameras.)
This assuming same technology level. For the same number of pixels, the coarse pitch does better in low light.
Either coarse or fine pitches have advantages. I coarse pitch shows less noise per pixel at the same low illumination level. A fine pitch shows more noise per pixel and also more detail. A fine pitch on a smaller sensor costs less than a coarse pitch on a larger sensor and the larger sensor might require a more costly and heavier lens.
So you're saying that with about twice the total light, the m43-derived image was still noisier? I am not sold on the idea that you knew what you were looking at, unless there was some large-scale fixed pattern noise in the m43 that did not cancel with the pixel shifts, or the m43 50MP image was gratuitously oversharpened at the pixel level.
I recommend the DPR Studio Comparison Tool because it includes not stats, but stats in visible action that includes differences in read noise character.
Put any 20MP m43 up against any 20MP FF at 4x the ISO, and the m43 holds it own, especially the new Olympii. Put the Canon GX7-III (2.7x crop) against 20MP FF sensors at 8x the ISO, and the 1" sensor competes well even with the Nikon D5:
The granularity of ISOs, pixel sizes, and sensor sizes is too coarse for us to simulate the "same total light" in the 4 windows in the tool for all possible camera comparisons, but we do have the same ratios available in some cases, and when you compare same total light for the 4 windows, the visible noise is virtually independent of sensor size or pixel size, with the same amount of light. The pixel area ratios for the Canon R6, R5, and R7 are almost exactly 4:2:1, so if we use those numbers to scale the ISOs to receive the same total light per pixel, and throw in an older 20MP FF (6D), we get:
Based on your history, and what I could expect as a range of possibilities, I would bet that you forgot to normalize something.
I just took a look at the two cameras in the DPR studio comparison tool, and the Olympus gives much less visible read noise at high ISO with the same total light.