First of all I thank you and respect very much your comments in this thread.
I don't intend to start a debate on APSC vs FF, because they usually don't end up with any conclusion, or everyone draws their own regardless of what we get to compare.
Theory tells us that a larger sensor receives more light, and therefore will have more information about the image. From there, technology has advanced a lot and these comparisons should be made on sensors with 'similar technology', with a lot of care.
The experiences I read (or see on youtube) about photographers who change to a larger sensor, either APSC->FF or FF->MF, are that they find the dynamic range is better, and that the color information is richer, and that is noticeable when editing and manipulating the image (for example Manny Ortiz has some videos about this).
There is another point about sensor size that should have some influence. An image projected on a larger surface will have smoother transitions from light to shadow, that should be noticeable in the image. I haven't found any article talking about this. I made my own tests but, ok, these are my tests. We often do these tests to convince ourselves that we have made a good purchase.
But I am not an expert in sensor technology or physics, so this is just my opinion.
As for my own experience switching from APSC to FF, I have found the improvement to be greater than one step, but I am comparing cameras of the K3-II and K5 generation against a Z6 II (more modern), and so they are at a disadvantage.
I have not been able to compare the more modern Pentax cameras such as the KP or K3-III. But when I was considering the Z6 II, I obviously had to compare them somehow.
To make my comparisons, I used PtP charts or the DxO reports, but I think these should be just a reference, because they tell only a part of the story.
I also took an eye at studio tests from sites like dpreview. Yes, sometimes they also seem to be questionable, but you can compare the same image at different ISOs, and then you can get an idea of the real performance. Another source for comparison IMO is to download RAW images and edit them on your PC.We quickly get an idea of the actual noise, highlight and shadow recovery, how detail is maintained when you raise the ISO, if they can be easily edited, etc.
This is more or less what I was doing during months when I was looking for a new camera.
For the ZF I think we should take the Z6II sensor as a reference, as the Z8/Z9 stacked sensors are known to perform less well (regarding ISO), compared to non-stacked.
If I compare the Z6 II sensor against the K3-III, the PtP chart tells me that the K3-III is much better starting at ISO 200 (the K-70 is 'worse' on the chart), but if I compare pictures at different ISOs (dpreview site) I see that the story is very different. I at least don't see that graph advantage in pictures taken with both cameras, rather I see that the advantage is for the Nikon. What I did find similar, and maybe better, was the K1 sensor, but... you can't have everything.
That's why I say that these graphs should be taken as a reference, I would at least advise to download a batch of images and edit them at home.
Anyway, the sensor is only a part of a camera into a camera system, we need to take into account AF tech, ergonomics, build quality, lenses available, compatible 3rd party lenses, flashes or other devices, etc.
Regards.
