I don´t get it, this suggests to edit the images to double the size?(i did it and it doesn't work)
In full screen the screen it's the same size as large but it works fine. Why windowed should be any different?
most of your default images are prearranged to be 512x384, the gba size. Not to mention for most scripts the viewport is defined to be only across that 512x384 viewing area. Your images aren't defined over that viewport so they're cut off because they're set to float.
If you open up other screens you can see that some are assigned to coordinates rather than ambiguous centers so they shouldn't be affected.