This is a difficult story involving a lot of image-processing maths.
- You simply have algorithms that rely on pixel-per-pixel data like blurring or sharpening. You must expect output data to be depending on scale. In modules like D&S we try to minimize that effect but it can’t be removed completely. That’s life (or that’s maths)
- If you scale at some point, you always must take some scaling maths into account. (Read about nyquist for example.) If you look into the feathers structure you will run into problems very fast. Also read about “moire” here.
- Some scaling algo are more prone to “overshoot” (lanc variants are one example)
- If you talk about preview, i guess you mean the thumbs in lighttable view or the top-left pre/overview? If you want to see what you will have while exporting (leaving out jpeg compression artefacts again) you have to zoom-in to 100% to avoid any scaling differences.
Just some additional points to think about leaving color maths out of discussion here.