So I kinda stumbled upon this while making a small portrait mod, with a lot of help from a friend. The images were fine for me but for him, they were too small, like 50% of what they should be. Long story short, we figured out that, for whatever reason, when I saved the images as dds files with Nvidia tool, having mipmaps enabled was the cause of the downscaling. I figured I'd just put this in here for anyone else who was looking for a possible cause and solution to this issue as I couldn't really find any myself when looking into it initially.