Hi again!
I think the issue is in the import method. I don't know much about it, but I think that in unity you have to import pixelart in a specific way or the engine will scale it the wrong way. The windows viewer doesn't work quite good either with pixelart.
Here is a link with a blogpost from unity talking about pixelart: Pixel perfect Unity
It talks about a lot of things but I think this may be the part that could be useful to you: 