Comment 1 for bug 1460407

When the desktop scale factor is set to 1 instead of two, the wall paper covers the whole screen. However, that make menus, buttons, etc. too small to work with.

I assume that while determining screen size, the wall paper does not account for the scale factor.

The scale factor is set via dconf in the schema "com.ubuntu.user-interface". the property is scale-factor. Normally the value is {'eDPI': 8} in high res displays this is set to {'eDPI':16}.

See also: http://askubuntu.com/questions/472262/adapt-ubuntu-to-a-high-dpi-resolution-screen