Hi Jildert,
This is probably because the image is projected through your camera.
This is the default projection method for images loaded into the image map shader.
If you set the mode to "Plan Y" (side X and Z) then it will be mapped to the surface of the planet.
The size is set to 1x1 by default, which means 1x1m if you project in Plan Y.
(with camera projection 1x1 means 1:1 projection of image into the camera 'frustum')
Hope this helps!
Cheers,
Martin