The pixels on a display are a grid. Either you're using the native hardware grid of the display, or you're mapping some smaller virtual grid onto the hardware grid by scaling the image up. Scaled images can be either an integer value (e.g. 1 pixel to 2 pixels), or a fractional value (e.g. 1 pixel to 1.25 pixels).
If you're scaling the image by a fractional value: e.g. 1.25 (125%), then the OS has to do more anti-aliasing, which is a kind of 'smudging' color over several pixels to fake fractions of a pixel.
Windows scaling uses less anti-aliasing, and rounds things to the nearest pixel, making the image sharper, but less accurate in size and position. MacOS's anti-aliasing is more accurate, but uses more sub-pixel artefacts, which can look blurry, particularly on lower-density pixel displays.
In short: you can't select a fractional scaling without some loss of sharpness.
Apple's default 2x scaling for Retina displays should yield the best results. If you're using a non-Retina screen, then using an integer scaling will yield the sharpest results, but bear in mind that the pixel density isn't as small.