This is expected, depending on how the software developers made the software that you're using to view/preview your photos in.
If you look at those same photos in another software at the same exact size, you may or may not see the same effect.
It is one way that a computer can calculate where to put pixels, and what to do with areas that share different qualities of pixels, when an image is scaled. Ask more questions and I'll tell you more, but if you research "scaling interpolation" you'll probably find enough.