Maintain resolution of wallpaper?

[Snoops8]

So here's the rundown, and I don't know how to do this one thing that's been bothering me.

I have a full resolution wallpaper (3840x2543), and on my desktop computer with a 4k screen, I have it set to "center" so that the wallpaper takes up the whole screen (some peripheral pixels are lost, which I don't mind). On my older laptop, windows syncs that same wallpaper to take up that whole screen (720p). However, on my Surface Pro 4, it only takes up maybe 4/5 of the screen, and when I checked the file size of the wallpaper, it looks like Windows automatically downgrades the resolution to such a small file it won't even fit the Surface Pro 4 resolution (2736x1824).

I've tried setting the wallpaper via IE, setting it as a PNG, making sure the photo is the exact same resolution as my desktop monitor, making sure it saves as high resolution, to no avail.

I suppose I could use "Fill" mode instead of "center" mode for the wallpaper but that doesn't solve the problem, because while it will fill the computer screen with said wallpaper, now it's using a wallpaper with a much lower resolution.

Any ideas?