I have a 14x2 spritesheet which has 125 Kb in raw .png. But as soon as I compile that to .xnb, it mutates to 4 MB.
So my question is: Why is that? And how can I fix that?
I believe images in xnb format are uncompressed so image file size is purely a function of image size and colour depth.
Width * Height * Depth = Size
If your image is 1024px square at 32 bits per pixel, it'll be 4mb in size.
You could try using the DxtCompressed option, in the solution explorer > content project > image file > properties > texture format - change Color to DxtCompressed