The **Genki Dama** (元気玉, *Spiritual Energy Sphere*) cannot be created by someone with an impure heart and mind. This implies that you use it against evil. It's positive energy that you use to get rid of the evil that is poisoning your world. 

Still, this does not mean the Genki Dama is not powerful enough to destroy a planet. After all, King Kai says:

> [...] If you can draw so much destructive power from a ball made on this small planet... ...Imagine what you can do with a Spirit Ball formed on Earth! If you can also learn to tap into the astounding powers of the Sun... Well. Just be careful. Or you may destroy the very planet you're trying to protect!" 

So I suppose this means that Goku gets *just* the energy necessary to kill Frieza (or whoever was there) and not the planet itself. He's Goku after all, he supposedly can control his energy.

Piccolo doesn't have such a limit. He intentionally destroys the Moon with pure, mindless power.