Fat Buu Super Kamehameha, mistake from Toriyama

[DannyDBZfanforever]

I think people think I'm exaggerating, but Super percet Cell said that he can destroy a solar system without any problem, but in the fight against Goku Super Saiyan 3, Fat Buu throws a Super Kamehameha, and this attack only destroyed 1/10 of planet earth.Whereas we can put a million earths inside the sun.
Mistake or something else.

[Hitiro]

The attack may have lacked intent or was formed in a way to prevent planetary annihilation. Because they can manipulate their Ki to do that as Vegeta demonstrated with the Final Flash. It may also have decreased in power when Goku hit it or possibly the way it hit the planet caused it to do that. There are numerous reasons that can be surmised as to why the attack didn't destroy the planet. It's not like Boo revved the Kamehameha up either so we don't know how much power it even had. It could have even been below Cell's for all we know. If you go by what a Kamehameha usually does on contact with something it doesn't usually explode. So perhaps Boo made it so that it would explode on contact thus reducing the effect of the Kamehameha which is usually a beam that just blasts through things.

[Eternal Super Saiyan]

The angle of which the blast struck at prevented a larger area from being wiped out. Final Flash as said above, was not aimed directly at the ground from above, like someone would otherwise do if they intended on blowing up a planet.