In Windows when you set a wallpaper it (sometimes) silently transcodes the one you selected into a new, smaller version. It doesn't always, there's a heuristic, but it's assumed it happens to prevent people from selecting a 1TB terapixel photo and have it destroy the machine.
Anyway, since it transcodes the WP into a JPEG, it has the ability to select a compression ratio. That ratio is pretty famously < 100% and as a result there's some degenerate cases where a wallpaper that looks good when viewed in the filesystem looks terrible when set to the background.
I've seen machines which needed to swap to show the desktop, because the wallpaper was a high/true-colour BMP and it was like half the memory the machine had!
