I would check the position of the backdrop. Check the size of the foreground planet. Last, check the size of the heavily displaced planet. You could get the same look with smaller scales.
I have not played around with doing space scenes too much. My advise may no be the best available. To start, I would get my foreground planet ready. Then add a second planet. Instead of rescaling the 2nd planet, I would bring it closer to the 1st planet;yntil it fills the sky as you want it to. Then I would work of the surfaces of the 2nd planet.
Terragen 2 has a large 'universe', but it is still finite.