Space-VURD

On a completely different scale/note from my last post, a thought-experiment has been rattling around my brain, about what would happen if you scaled down a planet's diameter but kept the same mass/gravity. That led to discovering this fine gem, which is ostensibly a "how much energy would it take the Death Star to blow up your constructed world" calculator, but also gives numbers for things like planetary diameter at different densities and the like. Unless something's wrong with the calculator, however, I don't understand why holding surface gravity constant at 1g while decreasing diameter would result in a decreasing mass, as well as increasing density. What am I doing wrong here? It seems to calculate fine for the inner terrestrials, but if I decrease diameter to 10,000km and hold gravity steady, mass decreases by almost 40%!

That seems impossible, unless I missed something in elementary physics... ::edit:: and of course, I did: Newton's Law of Universal Gravitation, and I screwed up gravity and gravitation. Thanks liquidfun!