He is strong enough to blow up the sun
Goku fired a 10 times Kamehameha at the sun in GT and it did nothing. Wrong.
Also it states very clearly when he is fighting Buu on earth that he has to refrain from using full strength because the earth can't handle it. When they go the other plant after buu blows up earth Goku says Alright now I can fight at full strength. Watch DBZ again
As I remember it Goku said he held back to allow Trunks and Goten to have a crack at Buu. It also makes no sense. Super Vegetto didn't destroy the Earth and he was much stronger than SS3 Goku. So was mystic Gohan.
As for the blast they can shoot out many different types of blast, some blast are made to only do surface damage and some are made to pierce in for depth
This a popular piece of Fanon among the DBZ fan community. It allows them to believe that Dragonball Z characters routinely fire planet destroying blasts around but hold back the damage somehow.
Unfortunately this has no basis in the fiction. Any time a planet busting attack is used it is indicated by the character themselves or someone else that they have put more energy into the attack. If this theory was true neither Goku or Vegeta would of felt it was necessary to fire potentially planet destroying blasts at cell if they felt they could have the same power with a lower area of effect.
A nuke does a huge surface area damage but doesn't create a huge crater, but a bunker buster creates a huge crater but little surface area damage.
That's because Nuclear Bombs are detonated well above ground level. Which is why there is no crater. Little boy was detonated at a height of 580 metres.
For the effects a powerful explosion look at the results of Asteroid impacts or underground nuclear testing. there is always a huge crater.
Case in point: http://en.wikipedia.org/wiki/Image😖edan_Plowshare_Crater.jpg
Do some research guys when you want to mouth of about a theory based on science. This is almost as bad as the idiot who though light can get to Saturn in less than four minutes. It's the internet guys, research isn't hard. It takes 80 minutes for light to reach Saturn. Yes I'm talking to you Carver9.