I haven't a clue who said it, but I remember reading somewhere that when dealing with SF or fantasy, you've got to show differences instead of tell them, i.e. say the door slid open with a whoosh instead of saying that the electronic door opened. It comes down to presenting the details as common, every day facts instead of announcing how brilliant and clever you are with description that, if the world was real, would not be necessary at all.
I kept that in mind with my most recent revisions. I gave the fantasy world a small, red sun and told how the whole planet is smaller.
Then I stopped. What if a small red sun means the sun's about to go supernova or a smaller planet means the gravity's all whack? I immediately emailed a physics expert friend and asked, and checked my email about a dozen times until she replied. Fortunately for me, a smaller sun would just be a little colder (and can be fixed by making the sun be closer to the planet) and a smaller planet would be OK as long as the core is denser. In order to be accurate, I feel as if I should at least mention the core, and my friend had a great idea in linking the core to the magic of the world.
Which just goes to show, you've got to do some research, even if you make the whole world up!